I bought some jacket nylon on clearance and used some left over foam from a seat cushion I made some months ago to make a wipeable pad:
I first split the foam in half so it'd be thinner. I didn't want the foam to be too thick for suffocation risks, and it gave me twice as much surface area to work with. I attached 4 pieces of it together very loosely with an overcast stitch and then moved the pieces to lay flat.
The nylon casing itself was pretty simple - I just cut out two rectangles about 1" larger than the foam and sewed 3 sides together and most of the 4th, leaving a gap big enough I could turn it right side out and put in the foam, and then closed the hole by hand. I then ran it through my machine at each point the different batting pieces were joined, which both helps keep the batting in place and allows me to easily roll the pad for storage.
Since nylon isn't very comfortable to lay on, I also made a little cotton cover out of some monkey fabric. I also then will be able to remove the cover and toss it into the wash easily.
This was made almost identically to the nylon case, except instead of closing off the 4th end, I made one side longer than the other to be able to wrap around the pad and hemmed the raw edges. The longer end is then folded around the back and helps it stay in place both when laying flat and rolled together.
I have a couple more pieces of fabric I'd like to make into additional covers, so you'll likely see those at some later point. All-in-all a pretty successful evening I think. :)