First I have to apologize for my sometimes not technical english (second language... I don't know all the terms, google gave me wedge for spacer

)
The wooden spacer goes in first (obviously

) then the foil, on my Patrik I have to wiggle it a bit to get it in too.
(it even still sticks out one mm,which is fine with me)
I agree about the tapper part to prevent a normal fin from coming up in the box if over tightened, but you have to consider the extreme force of the foil in action.
It will stretch the tuttle box (it's only plastic with foam around it) if you don't prevent it from going up. We've all heard those "crack sounds" when over tightening a fin, the foil will do worse.
where's bloody Barn when you need him !!
I'm not trying to prove a point, it happened to me on the hypersonic, it went up and forward !
I ended up with the two bolts sticking out behind my foot
The local shapers told me that you also have to ensure you original box is connected to both top and bottom of the board, I only have patriks and it's the case, no idea if other brands would have "non connected" boxes. (seems strange they would)
What I did for the hypersonic (as the foil strut had made holes in the box) was to insert the foil (greased and glad wrapped), drill two holes in the deck between the bolt holes, put two straws in the bolt holes, and pour microballon epoxy in the drilled holes to fill the gap. Now it's a perfect fit and there's no way the foil can come up (without ripping the board

)
Imax, of course you should try, it's such a thrilling sensation !! Plus you can then ditch all those 8.8/9.6 sails and go for a blast in 8 knts !
(but beware it's bloody scary at first !!)