Perhaps over thinking it a bit. But then you officially just joined the club. So is this just going to return right back into the tank ?
BTW: Silicone doesn't stick to those pick up tubes which come/w like under gravel filters neither PVC pipe. When you change
the formula to get different qualities from the finished product there is always a trade off. So what they do to the acrylic to
make it suitable for the tanks may make the silicone stick to it. You may wish to make a convex surface from one corner to
the next nearest corner, leaving a space behind it as a result of that curve. But I doubt it would have enough vacuum from
the siphon effect to keep it on the wall. This has about just under 1/4" holes in it in the #5 mesh size.
http://www.everythingplasticcanvas.com/plastic-canvas-mrl-y2.aspx?k2=c260
And if you cut it 1" longer(you can always trim it so maybe 1.5") than the 12" from corner to corner and just put the ends on the corners anyway it will bend outward to form a curve. That #5 mesh comes in a length that will be just over how tall your tank
is. So you can even cut a U shaped piece out of the top edge for the return line to sit in. Which really is just the top end of the
pick up tube. I didn't take "during construction" pictures so it'd be pointless to show it in a picture after finished but I have a
built in bio-only filter that opperates/w an air lift tube which I made in a corner of the filter with glass. It's the third leg of the triangle so to speak. I designed it with more at the bottom(tapered) than the top so the water would be unobstructed when
it went around the air stone. Which I used these for in coarse size which don't get clogged as fast.
http://www.petmountain.com/product/....com/product/aquarium-airstones/11442-504546/lees-discard-a-stone-diffuser.html
The smaller the diameter of the tube, the higher you can get the water to go up above the water line. But 1/2" is as
small as I've ever tried. Not for just the lack of aria of water that can go through but remember it needs to pass the stone.
For that reason, if I were to try this/w a plastic tube...the only thing I can imagine that might qualify as tapered(to let the
water go around the stone) would be to see if I could use one of those blue transmission funnels some how. Cut off of
course. And how to get it attached to a right angle piece ? But maybe you could cut off the top just where it would allow
a piece of PVC x 1/2" to fit in the end(at the top) because you could glue a right angle piece to that. Maybe just use a
piece of the transmission funnel that is only about 6-8" for the bottom end.
There I just thought my way through it while typing this...you can also.

 

thanks for the response and input. I don't have any clue about the actual mesh sizes, but I tend to grab a sheet or two of the plastic canvas stuff (I think it's used for cross stitch or something...) when I end up in a craft store. Just generally useful stuff for aquaria DIYing, and pretty cheap.

\_/ <- that's my ascii attempt at depicting the back half of the aquarium (top down view).

\=/ <- and that's my bad ascii attempt of about where I'm planning the mattenfilter. pretty much parallel to the back pane of the hex, with just enough room for the uplift tube and heater. I've thought about curving it, and I've got a short section of 1", but I think just leaving a ~1" gap, and ~1" of foam will be easier, and take up less space then doing a curved mattenfilter for what I'm thinking of. Though this all depends on the heater and uplift tube size, so I'll play around a bit more once I figure that stuff out.

As to the uplift tube, I'm not really looking to raise the water any, just circulate, I plan to have the outflow of the tube at the typical tank water level, so I'm more interested in increasing flow rate (or at least getting an adequate flow rate), head isn't really going to be an issue.

I picked up that very same set of airstones the other day. I was just poking around, and didn't really know what I was doing, but felt I should get something, and I figured that at the very least, if the airstones were useless, I'd at least have some airline junctures.

I've seen some plans where it looks like they just jam an airline adapter into the uplift tube, and not even put in airstones.

Anyone have any estimates on how the gph varies in say, a 1" pipe using an airstone, vs. just jamming the end of an airline in there? Although, I imagine for the setup I'm planning, it probably doesn't matter much (back to the overthinking things bit...).

 

If you aren't going above the surface, you will get more flow from a 1" than a smaller tube. There is more/w an air stone
but to what extent I don't know. But without, you never need to replace one. I do this in a tank where I use that soft blue
air tubing and it isn't as dependable about holding on to the air line as plastic tubing is so I don't want to have to fish out
an air stone from that pick up tube. You do get a bubbling sound from no air stone, but it's not much if you have a cover
on the tank. I mostly notice it after it gets quiet in the house at night. I try to remember to change that air stone every
90 days as little as they cost. Like I said before the course one get you more time before they get clogged.
BTW and FWIW: the hole size in those sheets goes...the smaller the number the bigger the holes. I think the #7 is most
common in craft stores/w 1/8th inch holes in it. I have seen a few people use these sheets as building material for parts
of sump filters etc and just use the very small tie wraps to hold it together/w.
Show us some pictures of it when you are finished it...

 

I took apart and used this for a uplift tube Amazon.com: Aquarium Biochemical Sponge Filter Fish Tank Air Pump: Pet Supplies

 

I took apart and used this for a uplift tube Amazon.com: Aquarium Biochemical Sponge Filter Fish Tank Air Pump: Pet Supplies

Ugly as they look, that filter is all most breeders use for their breeding tanks. Just multiples of in the lager tanks.
No hurt fish/shrimp etc. and easy to clean.
I would like to think that I scrounge most of my things which I use for DIY stuff but I'll admit I do buy some.
Like these for a lift tube.
http://www.petmountain.com/product/.../product/pond-plumbing-parts/11442-504991/lees-thinwall-rigid-tubing-clear.html
http://www.petmountain.com/product/...uarium-undergravel-filter-parts/11442-509816/lees-fish-saver-elbows-2-pack.html
But using parts off of something you already have makes more sense.