Reconstruction of a broken store bought paper holster in Fusion360 by tracing the old one as a 2D sketch.
You need 2 pieces of 4mm grommets or string of leather or jute to tie it together (whatever feels "authentic"). I used grommets, 100 pieces of them, punch (not needed since printer draws the hole) and crimper are usually 10-15 bucks in a set. You could use regular pliers to set it as well. I sticked to original 4mm size same as store bought item.
It could be printed as flat single piece or half folded version. Both will require you to drizzle hot water or use hair dryer. Hair dryer is safer probably but I prefer water from kettle. Please wear necessary safety equipment in this case, could be heat resistant work gloves oven mittens at least. Goes without saying, gloves are probably bad idea if they are not waterproof. It has a long ear for hanging on belt, I used my bare hands.
Just apply heat to middle section, gently bend it a little, apply more heat and bend more until you align the matching opposite holes. Once you use grommets or string to fix two sides to each other, put a cylindirical object like ladle/stirrer handle in the hole and pour more hot water over it slowly. This will cause material to yield and start forming a shape around what you put inside. Goal is to make it round enough so the prop gun can fit in. I used the actual gun itself for this since it was nylon and it could take way higher temperatures before softening and I poured it only from outside at that point.
I tried this out both in PETG and PLA. They have glass transition temperatures of 80C and 60C respectively, meaning you don't need to reach 100 degrees C boiling hot water.
Wear protection, be safe and have fun!