I did some searching for a belt case and didn’t find any from recycleable materials alas (Feuerwehr didn’t have Fairphone sized belt cases, most things I found (be it from felt, bamboo, reused bycycle tires or like Feuerwehr reused firehoses) where pockets or sleeves without a clip to attach it, so you’d still need a pocket to put it in.
The 3 belt cases I did found are on regular webshops (I was explicitly seeking for Dutch shops, or at the least with a Dutch interface and payment methods). What I’ve come up with, I posted in the Cases and Sleeves topic: