It seems fairly straight forward to me. You can print a card reduced on three sides by 0.3 mm and use some aluminium sheet the same thickness to cover those sides. It will cut with a utility blade and should bend sharply around the edges as it’s so thin, but still thick enough to have some integrity. That’s pretty much how original cards are made anyway.
https://www.ebay.co.uk/itm/254529945422
Just in case any ones interested here are two cases, one reduced by 0.3 mm and the other by 0.4 mm. The flat beams can be used on either side of the sheet to act as a bending jig.
a framework case reduc .stl (1.2 MB)