I'll have to get some precision tools and measure the new iSonic 121 fin slot.
I can use an adjustable parallel to measure at least half way down in the box.
I'll try to do that in the next few days and let you know what I find.
I would make it a priority, but Hurricane Earl is bearing down on my "hideaway" in Cape
Hatteras, so that gets my main focus until Hurricane Earl gets past the Outer Banks of
As far as "sanding" your fin box, I think you will be at it a very long time.
Perhaps get a file and be able to remove more material, and right where you need to remove it.
Another option would be one of the tiny wheel air driven belt sanders and make some sort of guide
so you simply widen the slot, not put tapers in it and goof it up.
Hope this helps,