I believe that A & J Canvas in Vanceboro, NC can make these. I was there getting a new enclosure and I saw a few of the hardtop lifejacket holders being made.
There are quite a few out there. If you google 't top storage bags' you will see them.
The hard tops are just a mesh material that has brass o-rings snapped into it. The mesh is laced through the O-rings to the underside of the hard top frame. One nice thing is there is a heavy duty zipper that runs across the material to allow you to quickly get to anything stored above. I like the A&J Canvas idea. They probably make them for GW.