Can anyone give advice on replacing mattress in the cabin? Cut foam mattress pad? What size to buy?
Are there custom size sheet available? I cannot find ready made online.
There are companies that will provide a mattress given a template that you send them. Ditto for sheets. They are very expensive.
I just bought a king-size memory foam topper, and cut it to size. I then loaned that topper to a local seamstress for use as a pattern, along with a thickness measurement, who then sewed me some fitted sheets. That is likely your best approach.