This is a cross post to the add-on gear topic forum...
So I like to keep it pretty low key on the water. Rod, line and a few flies. I knocked a few of these and keep one on my lanyard. Where I live (Evergreen, CO) has some great small fish water in town and I like to keep gear a rig in my car for some quick action.
These are made out of fly line holders you can get for free from any fly shop. I cut the edges down to almost flush with the inside edge of the "spool". I wind up to 3 lines around the spool and hold them in place with rubber bands. I keep extra flies inside the spool attached to foam, magnets and just floating free. You can leave a tag on to attach cord, key ring, etc. Pretty much just do whatever you want.
Start with any fly line holder that separates and has a pocket inside. I grab "Rio" holders from my local shop for free. They recycle whatever I don't grab, which is a lot.
I cut the edges off with kitchen shears, then clean it up with a small hand-held power sander.
Cut any foam or magnet sheet to fit the inside compartment. You can also glue something to the outside to cover the holes. Most of my flies are to big to fall out.
Wrap line around spool. You can put notches in the plastic to hold tags, attach flies to hold in place or whatever floats your boat. I like to rig a top water setup and a under water setup. Maybe a dropper rig and whatever is maybe working around that time. I keep mine in place with rubber bands off of asparagus from the grocery store.
Figure out some way to attach it to you. Or just shove it in your pocket.
Put whatever flies you feel you want into the inner compartment.
Go catch fish.