This is a pretty simple design for a quick fix to your costume Nerf gun holster problem. Let's begin. ..
Step 1: Inventory List
Cardboard, Worbla, or other thin but stiff material
double sided velcro
Step 2: Assembly
1 - Lay out your gun on the material and cut out enough for the front piece to wrap around it and overlap the backing piece on both sides by at least 2 inches. This gives you wiggle room and space for the belt loops.
2 - mark where you want the grommets and punch holes for these. you shouldn't need more than three per side and I used two sizes, but any will work. I liked the look of the small ones.
3 - Attach the grommets and punch the holes for the belt and leg strap.
4 - Feed double sided velcro (hook and loop) through the top belt attachment holes you just punched and loop them through your belt to size the length and drop of your holster on your thigh and cut to size. Next, feed the same material through the lower leg strap hopes and wrap it around your leg and cut to length.
5 - You are finished, unless you want to paint it. Wasn't that easy? This one got me out of a jam for my sons Mandalorian costume. Good luck and happy Halloween.