We wanted to replace the mattress in an RV several years ago. The mattress that was on the bed was about 8" too long for the bed and you couldn't walk around the bed. We looked at new ones, but the price was out of the question.
The mattress was fine, just too long. My wife is very handy, and said, "I can take it (mattress) apart, cut out 8" and put it back together", so that is what we did.
She removed the stiching along the top & bottom of the mattress and we folded all the material back about 2 foot. We then went up each side about a foot and cut the large wire on top & bottom, removed 1 row of coils, and slid the cut pieces up along the longer section of wire th the length we wanted. We then "hog-ringed" the wires back together. She had to remove some of the bedding material, but put it all back together just like it was. My folks couldn't believe we disassembled the mattress, shortened the thing and reassembled it,
My wife told mom & dad, "I like a challenge", and to that, dad said "I wondered why you married Ken!" I get no respect!