You can always buy receiver tube, several dimensions available, but it isn't cheap. I have taken the weld 'nipple' out of the inside of square tubing by using a section of all-thread and a piece of 1" plate to fit, but it isn't a project I'd care to ever do again. Might be easier to try that idea using the section of smaller tube for a broach shim and just pull the cutter through the difference between the two, but still not on my list of project anytime in the future. Six inches of tube might not be too bad if you care to give the idea a try. Another method would be to weld up either plate or angle for the outer tube.
Miller 251, Lincoln PrecisionTig 275, Miller DialArc 250 AC/DC, Hypertherm 900, Bridgeport J-head, Jet 14" lathe, South Bend 9" lathe, Hossfeld bender with a collection of dies driving me to the poorhouse, Logan shaper, Ellis 3000 bandsaw, Royersford drill press and a Victor Journeyman O/A.