The idea is to hide the heater & power sponge in the tank. I built an acrylic box out of scrapes. The box is 5" tall, the substrate will be 3". Some larger crypts should hide it. Careful placement of Driftwood should hide the wires.
I'm using a Finnex HMA 200 heater, Aquaball powerhead and 10PPI foam. There is a void ara under the foam to allow circulation around the heater element.
Larcat sent me some acrylic tube that I formed as outflow pipe. I won't know for sure until I do a test run next week. I might have to go up a diameter size on the pipe. I'm also sure I'll need to do some tweaking on the ht. of the outflow pipe.
That's an interesting idea. How do you seal off the powerhead end so all the water has to go through the sponge? Since I am making a Matten Filter for my tank, my first thought is that the sponge is very small, so the water velocity through it will be high. Are you using this primarily as a water polisher?
The top sponge is 2" thick. I may need to cut a thicker divider pc. Originally the thought was a pc. of foam to keep shrimp & fish out of the pump. That morphed into this. I've always been a big fan of sponge filters.