We added a memory foam mattress pad to our twins and I find it to be okay. The fact that any mattress is bigger than the platform is a PITA -- I sit on the edge, the pad moves and I growl...
XL college dorm length twin sheets fit these mattresses without modification. JC Penneys is a good source for XL percale & flannel sheets. Our daughter got us some of the Target Nick & Nora flamingo sheets -- I guess XL too because they sure fit well. We don't modify for the rounded top corner. I don't like the blinds because they're so easy to hit.
Search the forums on Bear Beds
to read about inflatable custom mattresses. Some people speak about local mattress factories being able to make countour end mattresses like this.