Another idea (which I wrote already 2 years ago for FP1):
pockets from feuerwear.de in the round:
Handyhülle Mitch aus recyceltem Feuerwehrschlauch
It even exists one fitting to FP1 (as I only read), each is unique, made from recycled firehoses, have a belt clip and are very robust.
Cheers, Robert
Edit1: webshop also in english
Edti2: Just some crosslinks to similar forum topics:
mostly for FP1: