My overcab bed is acceptable (to me, at least) with the standard cushions. Obviously that varies with makers and units and such.
My rear bed "mattress", on the other hand, was not very comfortable. A memory foam topper completely transformed it. Memory foam (and I think other foam) toppers are very easy to trim to size with an electric knife (the kind used to mutilate thanksgiving turkeys) or, with only slightly more effort, a serrated bread knife. I would not spend lots of time and money finding one made to fit if you can cut a standard size one down. There may be a little sewing alterations to the cover if you care to use it.
At least with my topper, it took a good while for it to air out. It had a very strong and annoying chemical smell for the first several days. It might be a good idea to unwrap outdoors and leave it in the sun for a day before use.
