advice please - IBD modeling experience is not that helpful...

I need some advice here.

I've been going for some hedge fund interviews but didn't do well. I got my investment thesis well thought out, and can do modeling reasonably well having spent two years in an M&A group. I usually got into the final rounds and am asked to build models on potential investments. I know about the accounting and the mechanics of modeling but have a lot of difficulty coming up with rigorous and defensible assumptions. As you fellow monkeys know, IBD modeling is a joke - we use street estimates or research reports and just throw them into the model. but this is not sufficient for the hedge funds that I interviewed with. They want to know HOW and WHY I make and justify the assumptions that go into the projections.

I'd like some advice here - it scares me to know that all these painful hours I spent in investment banking building and formatting models to make them look pretty, are actually not that helpful. I am looking for specific advice - books, contacts, articles, any resource out there - that can give me some pointers on how to actually make rigorous financial projections/assumptions that I can defend in a 3 hour stock pitch.