I made a rough design - the main purpose was to see how thick will everything be once fit in the case and if everything even fits. I will advance it more in the following days, for today, just rough design and mainboard mounts. So it seems that in order to accomodate everything, it has to be 249,5x170x20mm (which is pretty positive given the original tablet is 25mm thick and much larger). Important take is that I have about 18mm of room for the type-C cable. So I ordered this cable: https://www.aliexpress.com/item/1005003478288982.html?spm=a2g0o.order_list.order_list_main.5.259218022svDkG as it is L-shaped and the L-shaped side of the cable should be just small enough to fit in the case. I really don´t want to make the shape weird just to accomodate the cable.
Mounting of the display will be tricky as I don´t want to use two plates as the original tablet - one to hold it from the bottom and second holding plate from the top that screws into the case, that makes it very thick, so I figured that it will be better to print a hollow frame where the display glues in - it doesn´t make it less repairable because the display itself will be easy to replace, one will just print a new frame for the display or use heat-sensitive glue. The frame itself will be screwed into the case.
Some screenshots, I feel shy to post such unfinished work, but at any time, I am happy for suggestions, so I post things even if they are early in the design process, so that I can accept suggestions and critics before I put many more hours into it, just to find out it is stupid.
Tablet overall shape (without any holes for connectors, but inside there is already mainboard mounting done + venting holes):
Display frame (without holes for screws as I did not find a good place for them yet):
