There a lot of ways to make a sponge filter..... depends on what you have on-hand to use... powerhead or airpump.
I buy used equipment, powerheads and UGF uptake tubes are great and cheap. These powerheads just go and go. You drill holes in the uptake tube then wrap plain old filter floss around it then a optional filter pad. You can use a actual sponge, I have a big rectangle one I bought for a sponge filter..... it looks nicer. Don't ask me why I don't use it... Like most of the stuff I make, its held together with rubberbands.
Looks ugly, but I don't bother with ascetics. They get buried amongst the plants no matter how nice or ugly they look. Whats amusing is that plants can actually root onto the filter.
This pic is really old, but its the only one I have of this filter.