I really hadn't thought about it. I would guess that the cross section of the tube should be greater than the cross section of the bolt being used. Buckling must be avoided. I don't think this will be a problem with a relatively close fit of tube vs bolt. Steel tubing (not schedule 40 pipe) can be bought from many different sources in various OD's and wall thicknesses. I've been fortunate to find the right sizes for reinforcing axle slices and trailer axles. I'm sure that a bit of searching for steel tube in various wall thicknesses should get you close.
Are you wanting to use 1/2" hardware? Looking at onlinemetals.com I see .75 OD Cold rolled with .12 wall is available. This gives .51 ID. the cross sections is larger than the 1/2 inch bolt.
https://www.onlinemetals.com/en/buy/tub ... eckbox=on#
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Chuck.
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