Gather materials for the “Jig”; this means 6”x12” tubing (52”) and any extra heavy U Channel. If you don't have any laying around, you can use one drawer member, and one spacer on each side.

Clamp the vertical pieces to the "Jig" on each end, with extra U channel in between the tubing and the vertical pieces.

Make sure they are both orientated properly, that is that both roller slots are at the same end of the jig.

Insure the two verticals are flush at the top end.