A million years ago there was a diaper cover made by... oh crud, punkin patch? I think? Or Pumpkin Patch? Yes, I think that's it. Anyway, it was like Ana describes, kinda like a PUL diaper only without anything absorbent. Right sides together, stitch around leaving an opening, sew elastic in the seam allowances and then turn right side out. You could also do PUL + something non absorbent like microfleece or suedecloth.
The Pumpkin Patch covers were topstitched, and I never owned one but they were popular.
FOE is just so darn easy once you get the hang of it, it's all I reach for when sewing diaper covers.