All sheets fit with enough tucking. A 'universal sheet' with a pull string is a neat idea.
Sheets to fit odd shapes need to be custom made. There are too many varieties of berth shapes for each make, model and size of boat. Nobody would mass produce something like that. They would need to make many hundreds of different sheet sets to sell 10's per year.
A Custom upholster will work something up for you. Or find someone with a sewing machine and sewing skills.