I "recovered" some with cute diaper flannel patterned fabric before, and added snaps or velcro to them to make them fitted rather than square (or rectangular, rather). I cut an hourglass like shape out of the flannel, times two. Sewed the middle layered part of the diaper in one of them. Then put the two pieces together right sides together, sewed all but one side together, turned them right side out and edge stitched all the way around. Somewhere in their, I added elastic to the legs and a little bit on the back of the diaper, then put snaps on (or velcro).
Here is a website that has a bit more about it. I haven't sewn diapers in years, but this is where I had gotten the info from:
http://www.diapersewing.com/