I would recommend the FULL sized foam mattress in the back of the 5Gen T4R.
The one shown in this photo is a 4" thick tri-fold FULL mattress that has been trimmed to fit. Standard width is 54" and the width between the wheel wells is only 44". I trimmed the sections as follows:
Section 1 - no cutting - fits well between the doors (with the seat bottoms removed AND filling the void to support the head.
Section 2 - trimmed 10" in width total and centered the trimmed section in the middle section cover.
Section 3 - trimmed both corners adjacent to Section 2 (5" on the width axis and 8" on the length axis).