rear upright jig

[Bonzo]

4 Studs placed at 90 increments will do the job nicely.

Mark the inside radius of your tube onto your jig plate.

Divide the diameter of your chosen stud by two, Mark another circle on the jig plate, then drill & tap for the studs. 12, 3, 6, & 9 oclock.

If you don't have any suitable studs to hand, cut the head off of some bolts & then cut a slot in the top with a hacksaw or slitting disc in the grinder.
You'll be abe to screw the studs in with a flat screwdriver.

Long winded but it'll work fine