A mouse/keyboard tray based on this one on Amazon: https://www.amazon.com/gp/product/B01M2319VR/ref=ox_sc_act_image_1?smid=A3W3STWCK1O3NR&th=1

The joints should be connected with ¼-20 nuts and bolts.  The joint out near the keyboard tray should be 4 inches long to go all the way through. I used a heat set insert so the bolt has something to tighten into that doesn't spin freely. Might make a variant of the arm support later on with press fit for regular nut.  For now makeshifting a handle with a pair of vice grips but you may be able to find a knob elsewhere on this site. I plan on making a knob for the design in the near future.

Updates:

Uploaded newer version of track 8_25_22

Uploaded pictures of actually printed 8_25_22

Finally made 3D model for knob. 8_13_23 Printed in TPU. Might work in other materials.