I think the problem with SUP and long mast foils aside from injury and running aground is the length of the SUP means you have a huge lever acting on the mast especially while crashing. It breaks the mast.Hi,
The Axis, RRD, new Naish, new Slingshot masts will all fit. The masts all come form the same factory.
The spacing is 40mm from hole to hole.
Also if you have a big foil and a wide board, you ride with less kite power and a more upright stance, less need for long mast. But if you do try it, use a windfoil instead, its more likely to be strong enough.