I believe you are right.
You never actually gave any reason for not going down the pre-fab route, maybe you could elaborate.
In my circumstances it's simply ridiculous putting together a BoB I can't carry any distance, which kind of forces the issue - rather now than on that would-be fateful day!
I wonder how well a tarp like construction would stand up to a bad winter. I think 3 months might be pushing your luck - but I never tried...
On your last point on tools. I'm presuming these wouldn't be included in your BoB but stashed or "accummulated" as the months went on. Like you said its not really within the scope of this forum, I just hope theres a community left to build.
On that happy note...
Thanks for the help people, you've helped clarify my thoughts. Any further insights in to the subject would be good.