Designed to fit the backrest of my car, it has an angle just over 90 degrees. It can be modified and customized.

OnShape Link: https://cad.onshape.com/documents/d76607ab8260f7ae4262045d/w/860e863316f3ec4c2fa3cfa4/e/5837c9116c7fc65afc5b2f90

I printed it in PLA (but it would be better to print it in PETG) with a resolution of 0.2mm and an infill of 50%.
I used tree supports, but probably, someone can try to print it without supports; If anyone does, let me know in the comments...