I'm not using a painless kit, and I am not planning on using the stock heater components/panel, so this might not be the most applicable, but the images below are what I am doing to mount a Eaton Bussman 31M series PDM and some other components for wiring.
Front view, Drilled a couple holes into the inner/lower dash support bar and added rivnuts. I also have a connector piece not pictured that connects the top of the plate I am mounting everything to with the underside/top of the dash for more stability.
View of the back, no rivnuts yet in this pic, and an earlier version of the mounting plate but you get the idea.
I have been doing all the wiring with a spare dash outside of the car, along with my good dash in the car, it is really nice to reference points on the dashes to determine the length of wire I need to get to places, and not have to worry about pulling it all apart to wrap it later.