Designed these parts and printed them in TPU to protect two things - the finish on my cerakote weightlifting bar from the exposed metal on J Hooks and my knees from the corners of the safety bars.
I have a Rogue Monster Lite squat rack and the J Hooks are the standard ones that came with the rack and only has a piece of plastic to protect the bar on the bottom of the hook, so there is exposed metal for bars to rub against. The J Hook Covers in TPU have lasted me months with little sign of wear.
I also had a bad habit of bumping into the corners of the safety bars so I modelled a end cap cover that I also printed in TPU. Hurts a lot less when I bump into these now, but I also bump into them less I think because the red TPU makes it clear where the end cap is against the black flooring.
As far as printing advice - the more perimeters the tougher the piece will be but also less squishy. I recommend leaning towards tougher for the J Hook caps. Safety Knee Saver you can push things as far as you can for softness while still being able to print.