I think a donor frame may be the only way to go man-hour wise.
eBay Motors: 76 Airstream Trailer, Stripped to the Floor (item 120221825876 end time Feb-22-08 17:48:55 PST) <-- there are frames out there, but I couldn't recommend this particular one.
Not meaning to tease you at all, I just don't move very fast on my AS stuff BUT I am tempted to offer the frame from a 1972 29' that has shell damage I am slowly stripping for parts, though I have not opened the belly pans up on it yet. The trailer was in a low speed uncoupling at some point, it may be wracked beyond use other than cannibalizing.