I built my first moss wall last night! Rather than sandwich moss between two pieces of mesh I opted for an instant wall by sewing moss to one piece of mesh. This also allows for a much thinner profile which is important in a tank of this size - more moss, less plastic!
This guide is specifically for the Fluval Spec but can be adapted easily for any tank. Time will tell if it is successful or not!
Here's a step by step:
Amerimax Gutter Guard - This is a black plastic mesh used to keep debris out of gutters. It's flexible and lightweight and cheap! Mine was around $3 from Lowes.
Suction cups - I got these from Home Depot, $2 for 6. They're the medium size. Remove the metal hooks from them.
Measure the back wall of the tank (Roughly 6 3/8" wide by 7" high) and cut a section of the mesh. Cut out a notch for the filter outtake. Push the suction cups through the holes.
Put a layer moss on top of the mesh evenly and pat it down flat. Leave about an inch gap at the bottom as this will be pushed down into the gravel to help secure it to the wall.
Grab a beer and start sewing the moss to the mesh using black cotton thread (or fishing line). I tried to interlace the thread through each mesh hole, but it's fine to spread the stitching out further depending how much moss you have. I was just paranoid about it all falling off when I put it in water. It took me about an hour to stitch all the moss and it was nice and snug against the mesh when I was finished.
Scoop back substrate away from the back wall. Attach your moss wall to the tank wall with the suction cups. Don't cover up the filter intake grill (this area will need to be trimmed clear of growing moss to allow the filter to suck of debris). Once the wall is secure and in place, scoop the substrate back against the bottom of the moss wall.
(The black thing at top right is the filter outtake nozzle...it looks like the wall is bent over, but its not)
I'll keep this updated with progress of the moss wall and any modifications I have to make to it. Hope you find this helpful.