Thanks for the replies

I’ve had a look at the product I ordered and what do you know. It is for strat pickups. The website had tele rings but no black ones to match the rest of the hardware. My plan is to just cut the pickguard for now but I may look for a proper ring.

The dowel idea may work and after looking up j b weld I reckon it may work in combination to the dowel. I’ll drill the dowel as close to the edge as possible and then fill in any gaps with a fibre glass resin. Because I hand chiselled the cavity, the sides are rough so the resin should easily stick.

Is there any glaring issues with those plans?