I got a piece of DOM steel tubing from a local metal supply place and the OD was about 1.002" while the ID was about 0.758".
It's eccentricity is small (i.e., it's almost perfectly circular) so you wouldn't have to take very much off the OD...maybe only 0.002" or 0.003" to get a nice outer surface. On the other hand, the ID is a little on the large side, as I measured a factory Ruger Mk3 receiver to be about 0.752".
I don't know how critical the receiver ID is for reliable functioning, but I would guess that the more slop you have, i.e., the greater the clearance between the bolt OD and the receiver ID, the greater the likelihood of a failure to feed.
So I think 0.75" aluminum pipe may be the way to go, as there seems to be enough meat to get the OD and ID very close to the apparent factory specs.