nothing special

you can make it with one layer (or two) of PUL. I've done that - but I wouldn't do it again with just one layer of PUL for a bigger bag. The PUL is REALLY stretchy and when you have a big load of diapers in that bag it will stretch the fabric and eventually the PUL will tear from the repeated stress. The other fabrics add stability/support. But for a tiny wetbag (1 - 2 diapers) one layer would be fine.