You can put a short length of 5mm string through the holes above your shrouds with a loop on each end tied close to the mast. The less holes you have in your mast the better.
If you really have to put a backing plate in you just take the plate, place it on the mast draw around the hole in the middle, then file out till the raised part fits in the hole in the mast. Clamp it to the mast and drill both holes. It should fit through the big hole in the mast and you just have to use some wire, tape and a T-terminal to fettle it into place and rivet.