Certainly one can save some money by butchering a foam mattress down to size. Or, one can get a custom-cut foam mattress encased in fabric and edge piping that is just as functional and far more esthetically pleasing, albeit at a higher cost. We used replacement-matress.com but there are other suppliers. Our 78" x 58" (near queen-sized) costed $645, delivered. It is the most comfortable mattress on which we have ever slept. When we transition back to a dirt house in a few years we will need a new mattress. We will be ordering from these folks again. If I were buying a boat and found a hacked up foam mattress aboard I would want a price concession to replace it. Boats: choices, choices, choices. None correct, none incorrect and all very personal.
One caution and some advice. You will need a moisture barrier for the underside. The mattress came with a protector but that does not solve the moisture problem. Our mattress sits on a plywood platform (aft cabin). The hull is about 24 inches below. The temperature differential above and below creates moisture.. What worked for us is: (1) installed an insulating board, one-inch foil-backed (both sides) rigid foam from Home Depot, Lowe's, etc.; (2) on top of the insulation is dry-deck interlocking squares which provide an air gap. After having done so, we now get NO moisture underneath the mattress. Before having done so, at times, it became soaking wet underneath. Cutting the rigid foam board is easily done with a pizza cutter wheel.