Found one for $16 that is 23.5 x 13.5 x 12.5 which looks to have squared off edges that should fit inside a sizer while providing maximum internal space. The length is still too long but the position of the straps for the carrying handle makes it look like I could somehow fold in and tape on the inside to bring the length down to 16 inches.

http://www.campmor.com/outdoor/gear/Product___63209