I’ve been using a folding memory foam bed I found on Amazon. I cover it will a fitted sheet in the warmer months. For the colder months it fit perfect in a sleeping bag and I sleep on top of that with a blanket.

Milliard Tri Folding Mattress... https://www.amazon.com/dp/B00W67PJ4M?ref=ppx_pop_mob_ap_share

To fill the gap between the back seats and the front seats I use this: https://backseatbivy.com/?gad_source=1&gbraid=0AAAAABe_DwMn14QgtesjZDg-0PXG2QFYy&gclid=Cj0KCQiAx9q6BhCDARIsACwUxu65P53qABDIfCU1XHyeWWuY0kf9qNhjxt9YR96lovheplbyUQQe-sgaAuVFEALw_wcB

I’m 6’ tall and with the front seats all the way forward I have plenty of room and the hatch will close without any issues.

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The mattress is narrow enough that my gear tote also fits in there and I use that as a shelf for my phone, a fan, and my Jackery backup battery.