Diet has the right idea.

I built my shoulder rig using this: https://www.amazon.com/CAMVATE-Release-Manfrotto-Standard-Accessory/dp/B075XJ68B6

There are Arri, 501 and Arca QR versions depending on which QR plate you use most. Smallrig, Camvate, Kondor Blue, Tilta and more make a variation.