Sponge filters are terrific. With your water change schedule one or two properly-constructed large sponges would handle your tank beautifully.
I'm a bit confused about the comment that you are building them yourself using sponge and ceramic media. What's the plan here exactly? Why and how would you incorporate ceramic media? A sponge filter is just a large chunk of foam configured in such a way that water is drawn through the foam, either by an air lift or a water pump, and then returned to the tank. A Matten-type sponge filter has space behind it that some people fill with ceramic or other media, but the surface of the sponge itself is already so huge that I doubt the ceramic really does much for filtration.
Poret foam can be had in various sizes and foam densities; I buy the 6x6 chunks in a length of 19 inches, which you can cut to shorter lengths for different tanks. Use silicone to glue one or two 6x6-inch ceramic tiles to one end, closing it off and also weighing it down. The existing center hole will allow you to jam in a 1-1/4-inch PVC tube from the other end. Make the tube long enough to reach from the ceramic tile on the bottom to within an inch of the water surface. Swiss-cheese the part of the tube that will be inside the foam with lots of holes, thus allowing water to be drawn equally through the entire foam block. Drop in an airstone and you're done.
A couple of nice refinements would include drilling the Swiss-cheese holes at an angle towards the inside/top of the tube, which keeps the air bubbles inside the tube so that none of them bubble up outside of it. A 90-degree elbow at the top of the tube directs the outflowing water away from the foam, improving circulation in the tank. The airstone should be lowered all the way to the bottom, so that it doesn't block the tube halfway up and reduce the flow of water. Drilling a small hole in the top elbow through which the airline fits snugly helps keep the stone down deep where you want it; otherwise it might tend to float up and reduce your flow rate.
You can, of course, supercharge your sponge filter by using a powerhead or other water pump to move water through it. You can get some astonishing flow rates and filtration efficiency this way.
Finally, remember that a sponge filter can and should be thought of as a living entity in your tank, just like a fish, and it needs proper care. Clean it by squeezing and/or rinsing in tank water, to maintain the bacterial colony it supports. If you have two (good idea) then alternate cleaning them so that there is always one undisturbed sponge working its magic for you.
There are reasons why so many breeders and multiple-tank hobbyists...I'm talking about dozens or hundreds of tanks, not just 2 or 3...use sponge filters. With a large central air pump system, they are the cheapest filters possible, and among the best. A sponge filter has no integral moving parts, requires extremely low maintenance, provides good mechanical and outstanding biological filtration while being completely safe for even the tiniest of fry and shrimp. It even acts as a food source for these tiniest of critters by culturing large numbers of microscopic fauna on its surface, which the fry can pick at with no danger of being "filtered out" of the tank.
Downsides? Well...sponge filters won't impress gearheads and tech addicts. But if you keep fish tanks because you like fish, rather than because you like tinkering with hardware and colour-matching banks of PVC pipes and valves, you will love sponge filters.