using tubes as wheel spacers?

You would probably have better results using a couple hunks of 2x8 stacked to get the 3" thickness. Drill for the longer studs, mount with washers and nuts, start it up and use a belt or disk grinder to knock the corners off as the axle spins. If would be fitting with the half-asses effort and at least the load would be spread out evenly. Might actually survive if you stacked up some hardwood planks.