you left out whether you have hib braces. I found that the seat needs to be about 2"+ in the back to support the butt, If there are hip braces then the seat is 2" wider so it can be wedged into place and provide a little more support beyond the braces. Putting replacable hip pads down onto a seat that's as narrow as the hip braces can help hold it into place otherwise cutting it oversize and cutting slots for the hip braces to jam it into place will hold it without gluing. Mine have been 18"x20", one was 16"x20 but that was with the hip pads holding it down. You might consider making two wider ones and make the last one by gluing on sections from the other ones cut down from their 24" length.