What's wrong with buying one, probably of ferrite bobbin/spool type?
What about distortion, is this something that needs to be really clean, or does it not matter much?
The Q more or less rules out the lossiest powdered iron (#26, #52) but the rest are fine. Something #8 or #18 or any of the brand names (Kool-Mu, Sendust, MPP, etc.) will do. Gapped ferrite of pretty much any sort will do.
Tim