All parts fit together without any additional hardware. Mounting holes are sized for standard drywall screws. 

Printed with PLA+, Generic PLA settings in Cura.

Parts: 

1 Base rail - I printed with supports, and orientated the long dimension vertically. 

1 Pinion

2 arms - Print these first to see if your headset will work with them.

2 long dovetail racks with teeth

2 short dovetail racks

Assembly:

The pinion slides into the two sloped pins inside the base rail. It will only go in from one side.

Slide a long and a short dovetail rack onto each arm, and slide into the base rail. Try to engage the pinion with both rails at the same time to keep them equal. A small raised bump on each side of the base rail keeps the arms from sliding out of the frame when extending.