Apply mould release wax to the inside of powerbox first Can use builders bog with fiber. I wouldn't think there would be much to fill! Tape off where you don't want any excess!
Powerbox grabs on the sides so I'd try tape first.
Very thin skims of any filler can be weak and flake off so will never be a top result unless it does have some fibre Auto bog with fibre in it would be best and advantage is fast too.
Any epoxy filler works fine...I have used hi strength araldite with talc added or you can get Epifiller which is excellent. Make sure you rough the surface up first and clean it well to remove any wax or grease.
