Nice idea! I like it!
I would go with the drawstring, and a beltloop would be good but not necessary. Color is good (stick to darker colors), and I would definitely trade for one. Would be great to store extra batteries, trinkets, etc, while on the hunt (hence the belt loop), and good to store the gpsr after the day when its not in use. Just be sure its nice and soft inside.
Good luck!
Eric