[Chris152]

Quick update - we've been slow, mainly because I don't know what we're doing. First try at the internal profile was poor - we used the steel pipe I bought but couldn't really clamp it down strongly enough, and we tried the 90 degree bend around the pipe by lifting the sheet, which allowed the middle section of the sheet to not bend as much. So it had a bulge. We ditched that plan and I planed the 15mm radius on a piece of wood and this time pressed the sheet down to achieve the 90 degree curve, which seems to have worked! All hammering was done with a block of beech between hammer and sheet, and to hold the sheet in place for the curve we screwed a bar through the sheet and into the wooden profile.
We'd already put the 90 degree lip on using angle irons (the flat profile of the sash clamps didn't hold securely enough at the centre of the sheet, on our first effort) before doing the curve. You can see in the end shot that I also had to plane away wood to leave an overhang so the sheet could be turned through more than 90 degrees to allow for spring back.
Now we just need to cut the curve into the section hanging vertically in the photos, to follow the shape of the rear seen from above when in situ. I'll practice on the failed first attempt (which you can see is still attached on th eopposite end of the sheet in the photos), and I've cut the curve template for the jig saw in ply so hopefully that'll be accurate.

Not sure if any of what I've just written will make sense to you, but hopefully the pics are explanatory!

Then it's on to the external profile which, we hope, will be relatively easy. Ha.