Devcon Aluminium Putty would work really well for this but if you don't have some on hand, a tin of this will cost as much, or more than the WO finder bracket.
This is 2 part epoxy putty / filler and a truly excellent product which can be machined, drilled, sanded, filed and screwed etc. It is a commercial grade product and it is very strong and durable, but it aint cheap. I used to use it for floating and bedding the barrels on my target rifles and found 100 other uses for it.
https://www.blackwoods.com.au/part/0...FQwmvQodnSwEyQ
Cheers,
John B