At this point I decided to try and reuse the existing quadrant first. So I sanded it down with my new rubber sanding mould and then gently pried it off.
PXL_20240428_150134242 by
The Pingus, on Flickr
Then, instead of expanding foam, I shoved a load of foam backer rod down the gap, and then I plopped some Toupret Fil Flex in the worst of the holes, and hopefully give it a top coat of fine interior filler later.
PXL_20240428_165404098 by
The Pingus, on Flickr
Now I need to get the panel pins out of the quadrant, but that's a job for another day.