We found the cushions and foam pas a liitle rough on our first sleepover in the Trill. Costco to the rescue once again. We got a 2 1/2" memory foam mattress cover that works great. It cost around $120 C$. Because the matress is a standard double bed (54") the wrap up the back covers all that snaggy bits that supoort the table (not to mention a little bit extra insulation for those chilly Atlantic nights).
We originally stowed it under the back seat. It went in okay, but "memorized" the shape of the hatch, so extracting it was a bit of a problem. It now lives rolled up in a carpet bag.