I really wanted to use these, unfortunately I couldn't get them to work... that said, they are still in my gym, waiting on my next move.
The problem is that you actually need some height on a rack to fit them. I'm 5' 9", so let's say my shoulders are 60 inches from the ground. If I stand on a calf raise block, I'm at 66", for me to get up to tippy toes that takes me to 71". You're going to want an inch or two margin of error so you don't come out the top of this, so let's say you need to mount the top of the attachment 73" from the ground.
Unfortunately I work out in a basement and my squat rack is precisely 73" tall (and then has a pullup bar and crossmember installed, so I can't put anything that high).
I love the idea, and want to use this but did not appreciate that it just wouldn't work for a basement dweller.
That said, USA built and high quality, I corresponded with the owner during ordering and he was super helpful and informative. I'm just waiting to use it until I can get into a taller gym.
Performance
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Build Quality