I had a little left over fiberglass and an idea for a simple fork bag.
Here it is open
I used a one foot section of schedule 40 PVC pipe and two end caps for the body of the 'bag', and I rolled glass cloth around the pvc to form a sleeve. I cut matching holes in the pipe and the sleeve and some weep holes in the bottom of the pipe. To open or close the bag you simply rotate the sleeve around the pipe. It is held on with a web belt and 'D' rings like your helmet. I included a drawing to show the basic construction.
What do you think?
JB