I have modified a therm a rest with no problem. I left the mat out one time and when I returned something had decided to have it for lunch. The critter chewed on the sides right in the middle of each side areas. Both sides! I trimmed the areas picked off as much foam as I could to leave a 3/4" margin for gluing. Before gluing I used a scotch brite pad to clean any remining residue off. Next I took Seam Grip and applied to both sides and clamped the area with 2 wood strips. I also put wax paper on the nylon before clamping with the wood strips. Let dry for 48 hours, did a pressure test by over inflating and then layed on it for a length of time. Also checked in the tub filled with water. Worked like champ ever since. You could apply the same principle to modify your mattress. I have often though about tapering the sides in where you don't really need the support to save weight.