Those rattle kind of noises annoy the heck out of me.  As a kid in Michigan I heard stories about disgruntled autoworkers that would drop a coke bottle in the doors or quater panels - usually on Fridays, so the story goes.  And when I have to drill out pop-rivets on a sailboat boom to re-attach hardware, the leftover rivet material inside the boom always seem to want to rattle around to annoy me.

Here's an idea - if you think you can locate the thing pretty well, you might try drilling a rather small hole - maybe 1/4 or 3/8 inch, shoot in a couple of tablespoons full of epoxy or (maybe better yet) liquid nails, or expanding sealing foam (I vote for the foam - light and super sticky) or orther such goo-glue, then stop up the hole with some blue tape.  Then shake things around until you're pretty sure the squeegee has found your glue blob and is well enough stuck to stay that way for eterninty.  You can then pretty up the hole by giving it a very shallow countersink and a little filet material, then sand again.  And if this fails, you might have been able to drill the hole in the location you'd need to start your "window" cut-out anyway to remove the thing.

You board might have a "belly button" - which of course isn't an error or defect, it is a "feature."

   that is a great idea,thanks!

I am currently building the Kaholo 12.5 for my bride.  I am going to install the optiional round Beckson hatch as shown on pages 74 & 75 of the build manual.  Seems to me that if you did the same, you could turn the board upright and shake it enough that the squeegie would move to a position that you could reach it through the hatch.  In any event, the hatch is a nice option to have.

   That just might work, thanks .