Don't know if it was my "best", but i made an UL external frame pack by putty epoxy'ing (Loctite brand) some cut up carbon arrow shafts (into a rectangular box shape), and wrapping it with dyneema cord (and adding some cheap cut off pack straps).
Haven't used it much because on one trip, one of the corners broke apart because i didn't reinforce with enough of the expoxy (that became obvious once i examined it). But until that one corner separated, it was very comfortable and only weighed around 13 oz for a lot of space. Re: water proofing, just put my stuff in a garbage bag, like i normally would anyways. Unlike a fabric pack, it's not going to absorb much of any moisture when it does get wet.
In hindsight, besides reinforcing the corners more, i also would have used much thinner/lighter dyneema or spectra cord. I went way over kill on it, when i probably could have used fishing line.
For some reason, just have been lazy and haven't bought any more of the expoxy putty, but plan to do it soon.
I also rather like my hybrid Down--removable Apex insulation, quilt idea and mod.
As to, "And how/why did you come up with it?" I'm cheap, and i don't mind thinking outside the box but with a practical bent (usually, except for apparently my UHMWPE fiber windshirt or rain jacket idea). Quintessential Scottish heritage according to the stereo types--not that a put a lot of stock, in that kind of stock anymore...