20mm water pipe 90 degree locking coupler

A coupler that has 2 off vertical constraints (which can be fixed) and 1 off horizontal constraint which can be locked with a knob. The PVC pipes which it couples are 20mm water pipe (~26mm OD). The spacing of the vertical constraints is based on 2 off elbows flush setting the distance.

Used in the corelines A2 lightbox.
(http://www.instructables.com/id/A2-overhead-corner-LEDs-lightbox/)