There is rarely anything square on a boat. There aren't bed standards. There are often differences between vessels of the same model and year. There are hatches,compartments, and access points everywhere. Often, people ask me if they can just bring me their old mattress, cushions... to use as a pattern for the new mattress or bedding.. The answer is, no.
The only couple times I've had a fit issue, is when I or someone knowledgeable didn't get on the boat. Too much can lost in translation, worn down materials, and unseen obstacles. It's also not uncommon for something not to be working well for the boat owner and we need to have creative conversation to solve a problem. Custom furnishings are an investment. Wouldn't you want someone else taking responsibility for the proper fit? Another benefit; We keep copies of the patterns. Need another set of sheets? Mattress pad? No problem.