I was thinking of a Fly sheet on a flat bed trailer. 
Any “Bunkbed” size fitted sheet will fit a truck mattress
Asda for about £5. Sod these companies that want loadsa wonga.
Trust me. I used to have one in a DAF, and just stretched it a little to overcome the shape but it worked a treat. If you have a standard shape mattress, then winner winner chicken dinner.
Ken.