Update, and next question...
Phase 1 is now complete. As it turned out, with a brand new HSS drill bit and a corded drill (as opposed to a battery one), it wasn't so difficult to drill holes in the iron frame, so a lot of the previous discussion was solved simply by bolting on some brackets to attach the "overhang" at the front of the polycarbonate sheet to and to fix through it to the dome above that.
Now, phase 2: the upstand is metal and also gets a lot of condensation on those certain days in winter. As can be seen, it's roofing-felted on the outside and painted (Sandtex 10 Year Exterior Satin) on the inside. It's just the angled part (possibly clearer in the photo below) that's metal.
So, I'm thinking of attaching some polystyrene to the inside of the angled part. I've yet to go to Wickes to examine the stuff but I think something like:
https://www.wickes.co.uk/Kay-Metzeler-G ... m/p/210823So, assuming that's suitable, how to attach it? Glue would be one way, and I am thinking lashings of PVA might be suitable from the point of being able to easily cover the entire surface, so there are no gaps between the painted metal and the polystyrene for condensation to form between them, but as for the adhesiveness of PVA for polystyrene to satin paint ???
If that won't stick then possibly PVA over most of it for the filling effect and some more suitable glue around the edges and in odd dabs in the middle? But which?
Another thought would be to sandwich the polystyrene between the metal and a piece of ply (5mm? 9mm?) on the inside, attached by bolts drilled through to the outside (with suitable amounts of sealant applied). Obviously a lot more faff (the ply would have to be painted too), but I am tempted by this idea for at least the front, as that's where we climb out (from a temporary ladder inside) and without some protection it's inevitable that chunks of the polystyrene will be kicked off at some point or another.
Thoughts, ideas, suggestions?
Yes, replacing the foam seal is also on the to-do list!
Both photos copyright me!