The Ecto Goggles have two knobs on each side that were used for eye relief on the original AN/PVS-5 night-vision goggles. These 3D printable versions are based on the original knob designed by TheGoon on Thingiverse and the GBFans Side Knobs. I tried to mix the design of TheGoon's knob, with the functionality of the GBFans knob. 

The back of the knob features a slot that slides over a standard ¼" socket head capscrew. The knobs should pressure fit.