How to make an on-the-water-adjustable tool-less center Quobba fin for an US Box:
You need:
- a FCS Quobba fin (of course :-) )
- the FCS US Box adapter
www.surffcs.eu/products/fcs-longboard-box-adapter?lang=fr- a finjak
www.pelicansurfcraft.com/product-page/the-finjak
Alas, the Quobba FCS tabs are 1mm too close together, they do not fit

No worry, just file away 1mm:

And then it fits perfectly and snugly!
Beware to be more careful than me and not eat into the fin base :-)
Note that you should put the adapter in "reverse" wrt to is normal use, the tab with the screw hole in the rear, both because this way you will have a better leverage to pivot the fin, and the finjak will perturbate less the water flow.

Insert the finjak in the box.
Note: I have painted a line (yellow) to give me a handy visual clue to position the fin relative to it when adjusting it on the water.
Mine is at 20cm from the leading edge of the front fins (I like a closer cluster on long boards like this 9'0", I can go up to 26cm on shorter ones)

finjak ready

Positioning the fin

sliding the finjak against the fin adapter

Now, a sharp push, similar to the one needed with FCSII boxes, and the fin snaps in place.

The finjak has a very low profile (and a snug fit), that does not add drag.
Ready to go and experiment to fin the sweet spot for each conditions!