Ive got a small flat projector made by yauffey
https://www.amazon.co.uk/dp/B078HMWSNG/?tag=vwcalifornia-21
and, with roof popped and the ceiling raised, it sits nicely between the rear shelf and the upper mattress. It does have keystone horizontal and vertical but you cant change the picture size witjout moving the projector. However when its placed on the rear shelf, the picture size fits perfectly onto the poptop canvass. Picture quality is really good and although max volume isnt great it pairs really well with a bluetooth speaker.
Ive found the easiest method for vieing films is to download. Indoors and connected to the tinternet, you can connect to netflix, amazon and google play to download films etc. to the device and then 0lay the download in the van. Ita rechargeable too and lasts about and hour and a half, so no need to connect to anything in the van, although i have just bought a usb extension for it. My kids love it too but yes the neighbours think were nuts too!