Still - one is out and the acrylic stripped out reasonably easily. The alloy is a channel profile and is in two pieces, joined at each corner. This stuff is 6mm - but the channel would take 8mm.

More or less settled for smoked, uv grade 6mm polycarbonate. Fixing the temp bit of ply over the hole sorted out a good fixing method. Several large self tappers through the old holes will pull it in around the slight curve, and into place - you can then fiddle with the fixing bolts at your leisure, before releasing the self tappers. Hardwood trim covers the internal bolt heads.

The old stuff looked quite reasonable - so I gave it a little test. A not over hard tap with a pretty small hammer.

My apologies for the hammer - it's a 'greenhouse' issue model, with a custom repair by my late Dad I notice - anyway, it did the job.

This old acrylic just shatters - no 'flex' left in it at all. This was one sharp tap - about equivalent of throwing a stone, so am glad I took it out.

I'm also thinking that if 8mm material will go in the channel, then I may go for that.

I really fancied yellow - but it's hard to get in polycarb on a cut sheet. Maybe if I can get 8mm in, then I could get yellow in acrylic.