I think with the home built route, you might end up getting close to the price of just buying one.
Materials- 500-1000 (maybe more depending if you get overpriced special fittings, kind of canvas and whether you can sew as well, etc.)
Your time - priceless
but seriously even at min wage $8(california, but i hope your time is worth more) x50 to 300 hours = 400- 2400 depending on how much time you spend bs-ing around and reinventing the wheel (flippac in this case)
I think that patience with the used route may be the key in your situation. Maybe check out wildernests if you havent yet.
http://wildernest.proboards.com/index.cgi?board=wanted has some that will fit your truck on occasion.
If you do end up going custom, i think if you go side swing (wildernest style) you might be able to get away with just a really heavy duty hinge with a very long flipping bar (whatever that handle on flippacs is) and supports underneath the flipped bed instead of a tortion bar for either a forward or back flip.
My opinion is function over form, so no matter how ugly it is, its perfect if it serves you need. Might be somewhat of a theft deterrent if it is especially ugly. Good luck if you end up building it and i hope you post it on here.