I made this for my dad last christmas. He had a set of wrenches in the cheap plastic casing they came in from the shop. The casing was falling apart and I knew I could do better!
It is simply pieces from two old jeans sewn together with a strong elastic band salvaged from a discarded back pack. The red seams really pop against the blue and black denim! That's his initials there - as if it could be confused with someone else's wrench casing... ;) I just eyeballed the spacing of the seams and tested as I did each one. The wrenches come out easily, but not so easily that they fall out on their own.

(Btw, if anyone has a better name for these things let me know! Because I have no idea, certainly not in English..)