Go the plantguild.com and see the Vortex reactor. I used an old plastic spice jar for the cylinder, upside down, mounted the pump on top, ran the airline into the middle, and plugged the end with a kitchen scrubbie. The Rio 90 has 4 tiny suction cup feet so it holds everything up.
To cover the outlet, I've also used ballet-type netting from Walmart, and rolled up thin black sponge material from the pond pump. The netting was too fine, the sponge wouldn't seal well as it was thin and rolled up and left gaps. The scrubbie, actually half a scrubbie, is Ok but lets big bubbles accumulate on the bottom and then they release eventually. I just got a chunk of black sponge from BigAls for a few dollars and I'll use this to make prefilters for all the filters and a new plug for the reactor. I also got some larger netting but never got around to trying it. I just wad it up and stick it in the open end, but since there is no edge to hold it in, it eventually slides out. The sponge might do better, we'll see.
Like the Plant Guild reactor, most of the action is from the bubbles churning around in a circle since the input is off to the edge of the cylinder and this sets up a spinning motion. The sponge/netting/scrubbie is just to trap any bubbles that want to go out the bottom. If this was a bit longer I might not have even needed that, I've run it with no sponge and most of the bubbles wanted to stay in until dissolving. A few inches more length and it might have worked better. It is 4 inches long, maybe 1 1/2 inches wide, 5 or 6 inches would have been enough.
Comparing mine to the gravel vacs, which I've never built or seen first hand, I think mine has more spinning motion due to the offset water input and less gpm probably.
If I had it to do all over again, I'd keep the shaker top on the spice jar and put a thin sponge inside. And my pump airlocks when I turn it off, and the DIY gas keeps going (like during tank cleaning or short power outages). I have to tip it horizontal to get it going. I'm not sure how to make that not happen. I'm just too lazy to tear it down and try again.
With this next tank, I'm just going to buy one from Aquabotanic and get one that works right all the time.