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/)

Parts needed:

1. 2 off conduit_single_collar
2. 2 off conduit_cross_brace
3. 1 off conduit_cross_brace_plate
4. knob_M6
5. 1 off M6 Hex Bolt 20mm length
6. 2 off M3 Button Head 12mm screws with nuts
7. 8 off 4G x 9mm self tapper pan head screws

Steps:

1. Insert M6 bolt in knob_M6
2. Insert M6 nut in conduit_cross_brace_plate
3. Set 2 off M3 nuts in one of the conduit_cross_brace
4. Fasten conduit_cross_brace_plate to conduit_cross_brace with M3 screws
5. Fasten top and bottom conduit_cross_brace to left and right conduit_single_collar with 4G screws.
6. Insert 20mm water pipes in conduit_single_collars and between conduit_cross_brace_plates
7. Lock horizontal (if used as shown) water pipe with M6 bolt with knob threaded into M6 nut in conduit_cross_brace_plate

Pipe:
http://www.bunnings.com.au/holman-20mm-x-3m-press-class-12-pvc-pipe-_p4750039