IMO the hydro-sponges are the best design.
For sponge, you want an open-celled sponge that's firm enough to take repeated wringing out and still retain it's original shape. IME most open-celled sponges are flimsy. You also obviously need non-toxic. I've tried to find a generic sponge that fits these needs, but have come up empty. If you find anything please share.
The best solution I have found is to either buy actual Hydro-sponges (jehmco.com) or you can DIY from Poret sponge/foam. The Poret is expensive though (IMO) and only available in the US through Swiss Tropicals last I checked. I managed to pick up some unused material second hand through someone that purchased on bulk and from that I built my mattenfilters and a couple DIY sponges. The pricing does get better as the quantity goes up.
FWIW, the easiest way I've found to cut sponges is to get them wet and freeze them. Then you can even use a drill bit to easily make a hole for the uplift tube down into the center.
