Saint Crispan,

I had to throw the "newspaper" thing in just cuz somebody already had LOL. No offence intended. Anyway, I know where you are coming from. I'm doing my Challenger on the low buget plan myself and I think what I would do considering the lack of wleding resources is just what you are planning. In addition, I would suggest when you glue your backing in, to try to leave a little gap at the edges of the hole. This would in effect give the bondo a place to squeeze in behind the parent metal and act like a flange for the bondo patch. Also, as it was in deed part of you original question, I share the opinion that bondo sticks best to bare roughed up metal. Hope this helps and good luck on your project

Disclaimer: I'm not a body man; I don't even play one on TV But I've spent 40+ years DYI-ing. You learn what works and what don't after about 39 years YMMV