I actually sounds like your original fin is fitting in the way it was designed.
Tuttle and Deep Tuttle fins depend on the large surface areas on the sides of the
fin root to give the significantly better support that the Larry Tuttle design provides.
If it bothers you that you have to work the fin into and out of the fin box, you can
loosen the fit slightly using the following methodology.
Get a bar of soap (like you use to bathe).
Rub the bar of soap all over the root of your Tuttle or Deep Tuttle fin root.
Rub a little soap down into the fin box in the bottom of your board.
The soap will serve as a lubricant and may help the fin to slide in and out
If the fin root is still too tight, take a smooth single cut flat file and do the following.
Rub the fin root all over with the bar of soap.
Notice that the soap is whitish and dull looking to the eye.
Put the fin into the box and tighten the screws until the top of the fin root is fair with
the bottom of your board.
Remove the fin from the fin box.
Now examine the fin root.
All of the places where the soap has turned shiny and darker are the surfaces that
the fin and fin box are making full tight contact.
Use the flat smooth file a few strokes on the "high spots" (where the soap has turned
shiny) and then reapply the soap.
Do this a couple of times and your fin should slide in more easily without becoming
loose in the fin box.
Tuttle fins depend on the flat side surfaces for lateral support, so do not remove much
material from the sides.
Tuttle fins depend on the front and rear taper/angles to control the depth that the fin
goes into the fin box.
Using the soap as a "marking media", put the fin in the box and tighten the screws.
The fin should slide in fairly easily until the fin root is proud (above) the bottom surface
of your board by 1/32"-1/16".
That last little bit of depth is the "draw" on the front and rear tapers and this is important
to getting the maximum support for the fin root.
The 2 fin screws will "draw" the fin in that last little bit and your fin will practically become
part of the board.
If the fin goes too far into the board, you need to add a little epoxy or a soda can shim to the
front or rear (or both) tapers to set the fin higher in the fin box and get the required amount of
If the fin does not go into the fin box all the way when pulled in with the 2 fin screws, then using the
soap and your flat file, carefully (very carefully) file the high spots on the front and rear tapers a few
strokes and try it in the fin box again.
When you get the correct "draw" (~ 1/32" to 1/16"), your fin is fit to the board correctly!
When you remove the fin it may be necessary to strike the fin lightly from front to back to
get the "draw" to loosen up so you can easily extract the fin from the board.
This is how the Tuttle design is supposed to fit.
Hope this helps,