Davz, I just suggested the commercial takedown sleeve because it is already the right size and shape. Any pipe would likely work, just take more effort to cover up.
As an aside, the best shooting bow I ever made was the result of something similar. I had a couple of pieces of wood (ipe as it turns out) that were too short for splicing, but just right end to end. I put them together with a BowHingeŽ and got and unbacked Ipe ELB. Ron