Still working out storage and a few other things on my weekend warrior build, but wanted to report that a cheap tri-fold queen sized mattress with a california king duvet cover around it work great. I wanted the duvet cover to be oversized so I can still fold the mattress up and put up the 2nd row seats when needed. The back two panels of the mattress needed to be trimmed, which I found a box saw made quick work of.
https://www.amazon.com/gp/product/B07KWDBZZQ/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B00GH2Y1V4/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1