and I'm looking at ways of sealing it up. Standard acrylic sealant won't do it because of the size of the gap, so I'm thinking of...
1. Gorilla foam with sealant over the top.
2. Put a 16mm alu channel over the top of it, which will fill the 20mm hole to a level where I can use acrylic sealant.
3. Cut a 20mm dowel in half to form a plug, and seal up the remaining gap.
Any thoughts on that, or any other ideas? None are perfect, but I'm sorta leaning towards the gorilla foam at the moment, although it's horrible stuff to remove if I ever need to replace anything.