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While I admire your resourcefulness I can't help but think a Covert Bag from 5.11 or similar would be more functional, easier, lighter, and not open you to requests to play a few chords.

Here's the smaller version of the Covert 5.11
http://www.511tactical.com/covrt-m4-shorty.html

I do want to see how yours turns out. I have found that the foam, without airflow as you mentioned, doesn't set up very well.