Hi Lgvta,
We replaced our original 6" mattress with a Walmart Hometics 6" mattress. I would be worried that an 8" mattress will swallow you up. Getting in and out from the rear corner bed is hard enough. Before getting our replacement mattress, I had actually considered a very dense 4" mattress to reduce the "swallowing" and to increase the ease of getting in and out from the bed, but my wife didn't see it the same way.
Another concern with an 8" mattress would be the man-handling of it in the same tight quarters, like when putting on a mattress pad and fitted sheet. Our Hometics mattress is a whole lot heavier than our original cheap mattress. Heavier and thicker yet would be even more difficult.
On eBay, suppliers are selling very affordable 4", 6", and 8" thick foam. I wanted to experiment with various foam thicknesses and densities, but again my wife was not in agreement to do so. I had also thought of buying a 6" very dense bulk size foam, cut it to size, and zip it inside the Phoenix supplied mattress cover and be done.
CLICK HERE to review our replacement mattress experience.