This is a question I have never been asked in the two and half decades I’ve been in the mattress business, until yesterday.
A customer came in with two sets of specifications one for the a very thin mattress he had from a double size futon he’d purchased at IKEA years ago and the other for a somewhat smaller bed he’d also bought there.
He wanted someone to cut part of the double size mattress down to fit that smaller bed and then recover it with new fabric. I don’t know of any manufacturer that would take on that job and if they would they’d likely charge more to do so than buying a brand new custom size mattress from scratch.
He said he will continue his search and thanked me for trying to help and said he’ll be back when he needs a conventional sized mattress.
Nine Clouds Beds Mississauga mattress store is open 355 days a year until 6pm Monday to Saturday and 5pm on Sunday to help. Please visit soon.
https://www.nineclouds.ca/blog/can-you-stack-one-mattress-on-top-of-another-for-more-comfort