US finbox fin for iSUP - 7"or shorter

[SUP-Lurker]

My iSUP has thsi 9"center fin https://nixysports.com/products/nixy-9-center-fin. note the thumbscrew that is captive (doesn't get lost)

I want a shorter fin for shallow rivers. All the ones I found have a different type of screw and a sort of threaded square washer. Look at this example. https://smile.amazon.com/dp/B09XFLDN9D?psc=1&ref=ppx_yo2ov_dt_b_product_details. I'm not able to install such fin with that square washer, only without.

Is this some sort of different US fin box system? Some reviews mentioned it would be for hard SUP (not iSUP). Some videos make it look like that square thing should slide inside the finbox. But that doesn't work in mine.

Is there a shorter (like under 7") reasonable fin that is like the one I have?