Made mine too. It's fleece on top, mattress pad, then a cotton sheet under so it's not too "wobbly". It holds a bit by itself. Still have to buy the velcro to hold it in place
Measure, measure and measure again.. then another time to be sure. Take your time. For my sides I did like a box. If you want to see some steps : https://s574.photobucket.com/albums/ss181/Shurrim/Doby/Cage/?action=view&current=cageliner1.jpg
then the following ones.
