There are a lot of possibilities.

It would be no more complicated than the hold down set-up I installed. I rounded a 2×2 dimensioned to slide into the upper area, to give me something to mount hold-downs to.

You could either do as others suggested, so it’s just a narrower version of the type used on other fences.

You could make it with a T track on just the one side, which is away from the blade. It would have to slide on from the front. That way, it could have large nylon screws added, if you wanted it tighter.

I’d set it up so the hold down could be raised and lowered. Too, I’d make it easy to change out the piece holding material, so there would be no qualms about running it through the blade. Also, sometimes I find myself using a Masonite shoe for thin cuts.