those sucker-on silver bubblewrap screens for all the windows
the same can be done with spare suckers and a few cheap decathlon foam camping mats. silver face out for summer, inside for winter.
this page might give you some inspiration if you want to build a removable bed
http://www.webmontagne.fr/divers/Am%E9nagement-Peugeot-Partner.htmlthe tricky bit is how to hold the front plate(s).
3 piece designs require two side beams running lenghtwise, usually suspended on the corner of the rear plate and on some cord hanging from your safety belt loops.
2 piece designs are simpler, but only possible for a short bed or a rather long vehicle (tourneo connect 220, caddy maxi, etc)
Top it with stiff 50mm foam and you're good for much more than a few nights here and there: i slept on a similar bed during my 3 months canyoning guiding season last year, and when i got back i found my regular bed less comfortable!!!