Couldn't find a 13mm socket around the house so thought I'd try and make one with the 3D printer. It turned out pretty well !
I printed mine at 0.3mm with 100% infill for strength.
I designed it in 2 halves to avoid the need for supports.
Just super glue the 2 halves together and you're done.

Update :- Added a new version which can be printed in one piece.
This one has a hole in the bit to allow a screw to be fitted as most screwdrivers
are magnetic these days.