I use a big steel junction box on my hot plate for an oven too. But instead of cutting a hole, I just removed one of the tack-welded sides. Lacking a saw blade, I just bought a cover plate for the junction box and put that on. That side sits on the burner, and I put my mold and Lyman dipper in there to keep them hot when I'm not casting.
FWIW - I started out just filling the mold cavities with enough lead to fill them up, and was having fill-out issues. Then I started pouring it in and letting it overflow a lot, which seems to help force the lead into every corner of the cavities and I got much better boolits.