Here's something off the top of my head lol.
You sew up a skirt of the noseeum mesh, with elastic around the edge. The idea is you thread it through the guy tie outs of the Gatewood, and pull tight to seal around the perimeter (you'd have to tie off the bundle you're left with somehow, to keep it taut).
This way your Gatewood still weighs the same when you don't need the skirt.
Velcro would be quick and easy, but probably heavier? I don't know the weight difference between elastic and velcro off the top of my head. But half of the velcro is going to permanently be attached to your cape.
Maybe this is totally wack, but maybe it will give someone else an idea to chime in with, too.
Another thought is to sew in little loops on the inside to 'hang' the skirt from.