I bought a Dorma King Size memory foam topper for £170 for our B37. The length is ok but at the top it is about 12" (30 cms) narrower than the bunk on each side. However, this does not matter as it is unnecessary space anyway. The Dorma topper comes with a removeable (zipped) cover for access to the foam. I laid it out on the bunk and, using a straight edge, marked the shape of the bed. It can be easily cut with large scissors (stanley knife not too good). The re-sized foam can be put back into the outer cover and the loose corners tucked under.
I can honestly say that this has transformed our sleep comfort, even to the point where we now use the forward cabin for longer passages except in the roughest weather. The days of stiff shoulders and aching hips are gone