The mattress that came in my 'C' when it was new, was horrible. It was a short queen, on top of a platform that included the lid to the storage area at the foot of the bed.
I went to "The Original Mattress Factory" store near me. The salesman had me measure the distance from the top of the platform to the hinged storage lid and total length. They built an all in one mattress/box spring combo with a hinge where the storage box opened. Same price as a name brand short queen/box spring.
Unfortunately, they're only in 9 states, but it might be worth the drive. FL, GA, KY, MN, NC, OH, PA, SC, and VA.
BTW, to keep the storage box lid open while I rooted around, I bought a couple of crutch tips and a 6 foot dowel to match the inside diameter of the crutch tips. Easy to raise the lid and prop it open with the sticks while I root around for something.
Retired and visiting as much of this beautiful country as I can.