I’m also on team “put something under the brooder.” If you can get a refrigerator box, you could cut it to 8 inches tall and put the brooder inside. Would make cleanup easier since it would hold the bedding and you could drag the whole thing to the compost pile.
Maybe a few layers cardboard...
With ours, we used a heating pad set up (basically a DIY heat plate). They were in an unheated attached room, so similar to your garage I expect. Insulated but not heated.
I would say that for the first 2ish weeks, they went underneath the heating pad a good bit. After that, even though I...
I made @aart ’s version of the mama hen heating pad where it is shaped more like a heat plate and it worked amazingly well. I did have to hers the chicks under it a couple times before they caught on that it was the warm place, but then they LOVED it. You can tell you have the temps right by...