compression tube
 

What's the best material to use as a compression tube in an aluminum mast with a stainless steel through bolt for upper shroud tangs?

There could be reasons of wear and hole size requiring that the bolt holes (one on each side of the mast) be lined, bushed or reinforced, but you should not be holding the nuts that keep the stay tangs in place by compression. The ideal would be a pin through nut and bolt but if you can't work that accuratly a pin through the bolt just outside the nut will do. This is not something you need to torque.

That said, it might be that the holes were designed to be a bit larger than the bolt and what you're really inserting is a rather long bushing so it stays in place for both holes without further ado. It should come just flush with the outer surface of the mast on each side.

I'd use stainless.

Better yet...
 

Hello,
Instead of a compression tube, consider bushings. This does the same job that tubes do -- recruiting more mast molecules to take the load -- but with less mass up there, and no danger of corroding the tube to the bolt over time (one of my least favorite things).
We use shouldered stainless bushings, and mate them to tangs built with them in mind. On very thin-wall/high-load masts we might add a second, aluminum bushing around the first, and for extreme cases we'll also add a backing plate in the mast.
Note, no matter what you use, cut the bolt thread so that it stops exactly at the tang. That way there's no weakened bolt under the load.
Fair leads,
Brion Toss