I decided to go the route of mikedevelta and make 20mm frames rather than solid sides.
This gives you a little problem of how to get a screw in the bottom and side at each corner so we went with glue and a nail gun. Here's my carpenter pal (and occasional drinking partner) Bert with the first half of the first one.
We're in metric here and I was using 'the green stuff' from Wickes which is 1200 x 600 x 50mm, so I made the OUTSIDE dimension of the frames just about 1cm larger than this and cut slots in the fibreglass to drop it in and it then rests on the frame.
Here's a few with the frames complete.
I was trying to get some coloured muslin for the fabric but couldn't find any locally (and was too impatient for mail-order) but found something called 'scrim' which is like a heavy duty muslin but very breathable. Unfortunately it's also a bit too transparent...
I'll be installing them over the weekend so I'll have another photo update after that.