Hi Bob, Great finds for your build there!
We're going to have our mattress custom built here in town. To me, it's the whole point of building the trailer in the first place. To finally sleep on a good mattress while camping. A standard queen mattress is 60" x 80". You'll want extra room around it for bedding. If you make yours a bit narrower but keep the length, you'll be able to buy and use standard bedding for the most part. Are those dimensions your interior space(57 1/2 X 81 1/2)? If so, then I think a full extra long mattress would work very well. The extra 1 3/4" on each side and 1 1/2" end to end would allow you to make the bed with ease. Plus you can use off-the-shelf sheets and blankets.
Cheers,
Dave