I sail that kind of boards for fun & freestyle, old style, when the wind does not cooperate (<14 knots say).
My psychatrist linked my existential angst to this very problem, so I had too find a solution.
Solution #1 is simple, involved 2 x 6" Mitre 10 screws. Beaut is, you don't need stainless, as the board is guaranteed to take water much faster than the bolts will rust.
You just screw it the other side into any kind of fitting, it works.
(In this example, I screwed up and screwed... an old part!)
A nicer solution is the following, which involves a mast track bit like yours.
I cut out a 2" plate off an old heavy-duty aluminium hinge.
Cut it to length, and the width of the inside of the track.
A longish hole into which the usual bronze screw part sits. For thickness and avoiding cutting off the uni screw might hit plastic on the other side, I used a couple of layers of neoprene, from old mouse pads - that gives it friction too to keep it nice & snug.
And voila, you can now sail away with modern gear. It's resistant enough for railrides too - the ultimate test.
In my case, I use this to rig a modern Windsurfer One-Design sail, as well as a nice 2011 Dacron sail from Severne - lovely product by the way.
With this 2nd solution, the board will be drier than a pommies' bath mat...