FWilhelm,
You didn't say if you wanted to 3D print something, or just buy something. Additionally you were not clear what size you were looking for.
If you want to buy something outright, there is not much available...
PiTop
You could do an AIO, with a battery pack and external keyboard
https://www.canakit.com/raspberry-pi-4-lcd-display-case-pi4.html
https://smarticase.com/
But if you want to 3D print, there are lots of ideas... and if you don't have a 3D printer, you could prototype with cardboard, and then follow up with a print from Shapeways...
Here's my collection of UMPC's...
https://www.pinterest.jp/mike7620/umpc-ultra-mobile-pcs/
Mike...