Framebuilders - Halp!

Okay I'm getting in over my head here but I've decided to build my own custom frame, I've got almost everything worked out other than the tube for the fork and the bottom bracket. I want to know if I should fabricate, repurpose, or buy new/used? I'm confident I can install them myself but not so confident that these components can be made with hand tools.

Head tube and bottom bracket shell? Buy them. You have your hands full without trying to machine a BB shell.

Can anyone tell me if half inch OD carbon tubing is too small? planning on a double frame structure..yeah

for what?

using as my actual frame tubes, I saw an old KHS and I really liked the style

so you are running two top tubes side by side (like the Trek Earl just as the first example that came to mind)? Half inch should be good for that.

yes, and then the same for the bottom bracket to the head tube.. and then 1 tube for each seat stay and 1 tube for each side of the rear fork, I'm thinking of using the tubes then custom wrapping the joints in carbon fiber fabric to join them rather than going with the scotch weld bonding route... I will how ever be using that or something else to bond the bottom bracket headtube and seattube though. So does this sound viable? No kit not nothing.. home made frame jig to duplicated this frame I have.

I can't comment on your proposed joining method, but I'm not sure you're going to be happy with the down tube being two 1/2" tubes. To get similar stiffness to a normal down tube you'd have to go larger.

how much larger would you say? ideally I want this bike frame to be built all from the same size tubing...

okay terrible idea I know... but what if I did the downtube as bamboo reinforced with 1 or 2 oh 1/2 inch tubes internally and insulated with rigid foam?

Or what if you did the downtube as just bamboo? Plenty of people build bamboo bikes, several of them post on here, but you'd definitely have to go for more than 1/2" bamboo pieces for stiffness.