I finally went and bought a 4" foam pad for my v-berth at Walmart. It is a combination of 2 1/2" regular foam + 1 1/2" memory foam. I used a freshly sharpened regular kitchen knife to trim/saw it to size (worked great) using the original berth cushions as pattern and then folded and pinned the mattress cover underneath (rather than cutting and resewing it). Holy Smokes what an amazing sleep difference. It is firm but with a bit of give, no more bunching up problems like I had with thin egg crate foam mattress pads.
I left it as one piece to avoid eventually sinking through the middle seam, so getting at the compartments underneath is not as easy, but it is still possible (I only store seasonal clothes in there anyhow so not much call for getting in them regularly)...and the sheets stay basically in place while lifting the whole thing at once.