Interesting. I sleep in my wagon all the time on solo trips but it's a lot longer I guess, and almost perfectly flat. These are two things I verify before buying a vehicle that I find just as important as comfort and ergonomics of driving seat position.
All that work with plywood is cool but I don't like to ad anything that is not also useful in some other way, like if you could incorporate a camping table into being used for leveling the sleep area, or similar idea for dual purposing.
I actually cannot imagine cramming into a Crosstrek or Forester though due to length. Well, the newest Forester might be long enough as they seem to have grown over each generation.
For any of you, using the ceiling grab handles can make for great storage or hanging clothes or "curtains" or storing long things like a cross country ski and poles. I use a webbing strap with metal slide lock looped handle to handle, front and middle. Then I use varying lengths of bungee cords (the ones with the simple metal hooks which can be bent to desired shape if necessary), use the bungees along the roof line starting mainly at 2nd row roof grab handle back to little ceiling hole where many cars have a mount hole for cargo area load protection netting. There will be a little metal catch inside that is perfect for the metal bungee hook. This becomes great for drying clothes, hanging a "curtain", etc. Another can be strung from roof hole to cargo floor tie down ring. Use a dremel or drill to put holes in ideal locations for similar purposes, measure many times/test bungee lengths, then drill
The above is only part of my interior mods to make a 2008 BMW 535 wagon a mini-stealth sleeper
I usually have with me an ARB freezer fridge run off 2nd deep cycle battery and/or a mtn bike INSIDE with seat and front wheel removed all to driver side of center leaving sleeping half unblocked. Plus summer or winter gear bag, food bag, camera case, tripod, tent, sleeping bag, tool kit, etc.
Slept inside on last winter trip in ND -15F each night...