I ordered some rare earth magnets from China (hope they don't come with the pandemic) that are drilled and countersunk. The are 15 MM with a 3 MM hole. I am going to flatten one edge and place one at various places on my tang, barrel and breechplug to see if there is an optimum place for a peep.
The are supposed to be very strong magnets, 10 of them cost $2, shipped free.