After measuring it against Mike's car, I decided that it really needed to be about 30mm wider, so I used a paint pot and a pen as a guide to make a nice, level cut.
And then using an angle grinder, I cut it into two:
Using some 70mm lengths of aluminium, I made 4 joining pieces
And then clamped 3 of them (so I could rivet the 4th)
After rivetting all 4:
I then put tape on the outside, and used the bits I'd made earlier to fill the gaps: