[ERA Chas]

If you must do this job, because DMV mandates, OK. But the only right thing Sheldon Cooper said is to make your wife comfortable, not gut your finished car for this. This is a man that needs an instruction sheet for a nail file.
And if weather conditions really require a heater-it's too cold for the engine. Wear is not premature when it never gets to operating temps. But you're apparently past that or don't care.
Grommets are a good idea but the 'glass may be too thick. Try McMaster, then size the hole for the grommet OD. Forget files. Holesaw is great followed by a close-sized dowel wrapped in 60 or 80 grit paper. Finish with 220. Before that, get some old hand towels, wet them and place them on both sides of the f'wall to catch the 'glass dust tornado you'll make. Wear a particle mask for the worst of it.
Dan's brace is good.
Don't expect this all will do anything to keep anyone warm below 50 degrees. If you run in the cold, keep checking for milkshake in your valve covers and breathers.