Goniometer

https://www.amazon.com/BikeFit-7010102-Bike-Fit-Goniometer/dp/B004WNAI6M/ref=cm_cd_al_qh_dp_t $60 for a piece of plastic with no genuine ingenuity behind it.
Note the total length is 47 cm; this will not fit on most standard print beds.
Print Settings
Printer Brand:
LulzBot

Printer:
Mini

Notes:
Set layer height as small as possible to render the graduations as well as possible, & highest resolution.
Scale it until it fits on the bed.
How I Designed This

Reference Image

BikeFit Goniometer; used this to scale the length
See reference images above. I made cutouts to reduce material & help the user pinpoint the knee joint.
Arbitrary dimensions for the cutouts & sighting guides.



Assembly & Materials
4 mm bolt
4 mm locknut/nut
4 mm washer if necessary
Center hole is sized for 4 mm bolt.
Print the same thing twice; a 0th indent is provided & should be visible for most angles. Line this up with the periphery of the one in front.
Add numbers in sharpie yourself. Shaded regions are 35 to 25 deg on the Lemond goniometer.
Note that if you scale the print, which you probably will, the hole in the middle will no longer fit a 4 mm bolt.
Future
add graduations for measuring handlebars à la BikeFit Goniometer
move graduations around as users recommend
redesign & optimize for smaller print beds (6" x 6")
add numbers
forefoot measuring device