There are also 2 metal clips (with hole for screw) that clip on the top edge of the dash that hold the lens on at the top. (I have a couple of these) I also have a couple of the stud tabs, although most of the ones I've seen have a push on clip that takes a screw to hold clock to dash. I usually make the cutout with a hot knife but a coping saw would work, go slow.
Sam