Firstly a midge net to go across the opening. I sewed some ~10mm knock-off OneWrap (think I got it in Aldi a year or so ago) around the opening of the bag. It seems a good material for this since it's quite thin and therefore doesn't inhibit cinching it with the drawcord too much.
Hemmed a shape I cut out of mesh which I tried to get roughly to be roughly the same size as the opening (hard to tell since it isn't a fixed shape), preferably a bit larger but with the circumference as close as possible to match the OneWrap up. Turned out a little too small in the end, as you can see from where I've gathered the excess in one place.
The view from inside. (Yes, I've written my name inside in case I wake up in it and I'm too busy figuring out where I am and what the hell I'm doing there to remember what my name is... kidding, my partner bought an identical bag
)
It seems fairly well attached but it remains to be seen if it'll stand up to my nocturnal wriggling, particularly once the tape acquires bits of dust.
Also added a little webbing loop to the centre top of the opening so I can tie it to a tree/wall/bike to keep the mesh off my face:
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Same style but it's felted wool. Handmade, but not by me.