Well I hope I've learned a lot making this thing, because I sure made a lot of mistakes. I thought I was being clever leaving the fibreglass short in the mould so I could better form the joint with the back plate. But that actually meant a lot more work bogging up the gap. And the brushing gel coat which I thought would go on smooth as a top coat, ended up lumpy and furrowed and took a week of nights sanding to get it smooth again. And it's still not perfect.
I took the cut-out from the centre to the glaziers to get a mirror cut. Except I was lazy and didn't make the opening symmetrical and I didn't tell them which way the template faced and of course they cut it the wrong way around. So I've had to hack into the back with a router to make the mirror fit. Which means more sanding and painting to get it neat again. And at an angle, you can see the edge of the glass which is not good. And the whole thing has a slight bow in it so the mirror doesn't sit flush anyway.
The challenge now is to repair the mould and make another one without all the trouble.