HUMN wallet (https://humnwallet.com/) has wallets with good designs, in aluminum or carbon fiber. I wanted something similar but optimized to hold all the things I want to keep with it.
This is a front plate for a HUMN-style wallet with a slot sized to hold a Spyderco Panchenko dog-tag knife and a groove into which you can glue in my 3D printed cap for a bullet Space Pen. I haven't finalized my design and may end up larding it up with more items.
The design is not likely to be useful to you unless you, too, have a Panchenko knife and a Space Pen, but perhaps it will serve to inspire more truly customized wallet designs.
Updated: v2 increases some of the too thin dimensions around the knife pouch, reverses the fillets for better looks, and adds an elastic guide channel.
Updated: v3 adds a SanDisk low profile USB thumb drive holder.
Updated: v4 adds a 1 1/2" double ended screwdriver bit. There is a horizontal recess at the rear of the holder which allows the slot drive bit to sit deeply when stored. Remove and rotate the bit when using the Phillips head so that it extends further out.
Print with a single perimeter, 0.40mm extrusion width. Anything else is liable to cause problems with the bridge over the slot.