I share your marked aversion to putting holes in a boat
. However, as long as you aren't punching through to the outside, you should be fine... the glass in your keel
stub should be some of the thickest in the boat
, and you don't need to go very deep to secure a lightweight switch.
The old adage "Measure twice, cut once" was surely invented by a sailor, however. If you're going to make holes, make sure you get them right the first time! It's not impossible to patch, but it's no fun (IMHO).