Mwa-ha-ha, my plan worked! The slot came out A-OK. The polypropylene plug tapped out pretty easily with a mallet and screwdriver.
After I clean it up, I should be able to pop the magnet in.
My only concern is that when I’m casting, the bobbins are held in place with friction and pressure against the sides of the mold. If for some reason, the bobbin slid down just a teeny bit, it would ruin the whole piece. When I was using cylindrical magnets, I used the holes in my master to create these little nubs in the sides of the mold. The nubs fit into the holes in the bobbin and held it in place. Since I haven’t cut the slot in my master, I don’t have that option here. I’ll just have to make extra sure the bobbin is in place before I pour the epoxy.