I have 3 boards with Tuttle boxes, but one is noticeably tighter on fins than the other two. Anyone have suggestions about how to sand it out or otherwise widen it so it fits the fins the same as my other two boards?

Be careful. Is it the sides or the ends of the box that is tight ? Are the screws going in nice and easy. Sometimes the screw holes in boards are not right and can bind up things . If it needs widening, you will only need to remove the smallest amount . Easy to overdo it. Id glue a bit of 120 grit sandpaper to the end of a steel ruler or something similar. Take your time and go gently. Watch out that you don't take off too much at board level. Because Tuttle fin heads are parallel, you can push in one end down the box to see how you're going. Gently, gently.

I would not sand the box ,pick one fin that you would use on that board and sand the fin ,

A tight box is better than a loose one

Thank you gentlemen. It's the sides that are tight. So much so that the only way to get the fin out is to give it some good wacks with a glass hammer! Fortunately, that doesn't appear to damage the fin - even hitting it on the sharp edge. Yes, I'll be careful as you both advise. Gluing sandpaper to a bit of plywood sounds like an excellent suggestion, so that's what I'll do. Thanks again for the suggestions!

That's the best way to do it, but it's very easy, to sand the top of the box more than anywhere else, next is the bottom, and last the middle. If you can use something with a good fit in the box, that will help to keep it parallel. It very hard with something like a ruler, keeping pressure on the middle of the box instead of top and bottom, isn't easy. If you find that is happening, try just having sandpaper lined up with the middle of the box. It doesn't matter so much if the sides are a bit concave, but it's important they aren't convex.
And as already said, you won't need to take much off.

I would try some dry lube before sanding. Sometimes that's the difference, but if you determine it needs sanding do it bit by bit. do a little bit of sanding, then check the fit. It's very very easy to overdo it, even when it's just sand paper. Keep checking the fin head for markings where it's making contact, that's where you sand. As others have said, a tight fitting fin is far better than a loose one.

The custom fins I use have very consistent bases. The boards from Cobra, however, are all over the place so I use a fine cut bastard file to shape the boxes to fit the fins. That way all of my fins fit all of my boards.

As a side note, the problem with the sides of the boxes is that usually they are not straight, narrower in the center than on the ends. Also, the length is usually less consistent that the width. I use chalk applied to the fin bases to mark the boxes and determine where to file and achieve a perfect fit.

I use a file as well, much easier to get the surface flat. Don't run it perfectly perpendicular to the bottom of the board but more like alternating 30 degree angles.

A tight box is better than a wide sloppy one to hold your fin. Previously I have tried to slide my fingers in when it's dry to find imperfections but I found when the box is wet or moist- your fingers are likely to feel Imperfections in the box. Hopefully this tip will help your fin penetrate the box with ease but yet feel firm inside the box.

Agree with sub, try dry lube or even lithium grease before you alter the box. Often have to bed in items (keel/rudders/boards/even big winch bases) on race yachts and before you do anything that you can not fix up make absolutely 100% sure its the box not the fins or you could be creating a bigger drama. Most fins are Cnc cut and hand finished on the foil part so are seldom wildly out, however most boxes are cast or a combo of cast and layer up, some distort when bedded in. Most manufacturers use epoxy but big boats use spa bond for these jobs a far better product with less exotherm. Find some bright pink or yellow chalk and rub the bejazsus out of the fin base and then fit it, this will show the contact points and any high spots on either the box or the fin and hence where to very gently sand or file. In the end it's always going to be an average.

Yes but file only a little. You can also add more to a fin but to a box.... Shim your fin on your loose boxes with a plastic sheet cut out from a box or something.