More than a couple years ago (25? 30?), when I was getting into hunting I
made my own game bag for birds. I took some left over medium weight canvas
and made a simple deep bag with a shoulder strap. That's still pretty easy
as long as the wife doesn't catch me messing with one of her fancy sewing
machines. I think I sewed the original by hand.

I used the metal closer out of an old fishing bag in the top. It was
wonderful. If one of the birds I dropped in the bag started flapping again
it couldn't get away. When I opened the bag the spring/mechanism would hold
the bag open, and when I closed it the thing would be held closed. It
really was the best possible game bag for small migratory and upland bird
hunting. And even as bad as I shoot it would hold all my empty shells so I
didn't have to stuff them in my pockets.

My son expressed an interest in doing some bird hunting last year. I wasn't
sure if it would stick since he only dropped one bird and he lost it in
heavy brush, but he again expressed an interest this year. We have been out
three days, and we shot limits on two of them. (When I broke down and
headed out to my old stomping grounds on the second day.)

Now I am thinking about that old game bag of mine. I had stored it with
lots of other stuff from that time in my life in an old metal storage shed
at my dad's house. A metal storage shed that has long since blown away. So
I am looking at making a couple game bags like that one. I considered
buying a couple of those fishing bags and cannibalizing the spring mechanism
out of them, but they are all plastic now, and they don't have that nice
crisp snap to hold open and snap to hold closed like the metal ones.

Any suggestions for an alternative?

I've got some 0.080" stainless spring wire, and I am sure if I coil a couple
springs in it that it would make an adequate closer, but if it gets over
stressed on opening it will quit being a good closer. Also, there are times
when you WANT your game bag to stay open and it definitely won't do that
just made up as a simple mechanism.