I did send a PM to @anewrver but for the benefit of any future searchers I'll post my entirely way too long information on our mattress solution here:
So, we've used Mattressinsider.com to replace our current mattress in our FLX 2108ds and in a previous trailer. MattressInsider.com has the ability to custom order a mattress within their various mattress qualities. We chose to get a slightly longer mattress as we found with that we were a bit taller than the mattress that originally came with the trailer. (Not to mention that the original mattress was really not comfortable to us.)
One of us weighs over 200 pounds and we're both a bit on the tallish side. So the mattress quality we chose was the hybrid (combo of springs & foam) and it is a bit stiff at first but quickly becomes very comfortable. The mattresses ship compressed into a box and its worth opening the package, laying down the mattress and letting in breathe for a day or two to let it fully un-compress. That goes for any of their mattresses.
We had a 2 piece mattress for a previous bed and the separation fell right near our hips which we found gave us very poor support their and ocassionally the mattress would slip apart and you'd be on the plywood deck below on your hip or butt. Not comfortable. There are probably ways to make the 2 parts stick together better, but we never found one. So I was interested in getting a single piece, regular mattress.
I looked at how the 2108dr murphy space when closed was configured and saw that the space at the bottom was much wider than the space at the top. I also saw in a discussion thread that someone had bought a memory foam mattress of 6-8" deep and didn't have a problem getting it to fit into the space when closed. With that information, I decided to buy the Scardale Luxury Hybrid 54"x80" mattress with a 9" depth since we were not full-timing but planning to do many multi-week trips.
The mattress does hang slightly over the open bed space at night, but only by and inch or two and does give us a bit more length without really obstructing anything. At 9" high, I do use a step to get up on the bed.
We found it a bit difficult to "taco" the bed to put it away, so I grabbed an unused rachet strap we had for tying down motorcycles, golf carts, whatever that was long enough to encircle the length of the bed. We run the strap under the mattress and leave it there. In the morning we pull the remainder of the strap down over the bedding to the bottom of the bed, and pull the top of the mattress toward the foot of the bed with the strap so we can fold the mattress and use the built in cross strap to hold the taco shape of the mattress in place while we push up the murphy wall and close up the bed. Other than having some bedding sticking out when we forget to tuck things in before we close the murphy wall, the bed fits the murphy space just fine. You don't have to worry about getting the bed folded exactly in half, just at least fold down a third and it'll fit. When we have the fluffy, heavy comforter on the bed we might have a 1/4-1/2" of the wall sticking out away from completely flat in the murphy space but that's about it.
So that's about it. The strap idea came about because climbing on top of the bed and pulling the top end of the mattress toward you while crawling backwards to get the mattress folded for storage was awkward. My husband said the strap thing would never work, but I tried it anyway and he was pretty impressed and happy not to have to do it the other way.
By the way, I'm 65, have spinal stenosis and a couple of disc bulges, so the mattress issue is really important to me. And I'm able to fold the mattress with the strap trick by myself without straining my back.
Also, we tried just using a 4"memory foam pad on top of the original mattress and we both agreed that didn't work well for us. Although if you go that route, that will fold up into the murphy space easily.
I shopped around and found MattressInsider.com to be the least expensive of the rv bed options out there. Our mattress configuration cost about $750, so not cheap, but worth if for the amount we use our trailer.