I made Nick's footbrace following the dimensions of the top part carefully, but just used whatever pieces I had on hand when making the bottom.

I made it so that the slider was a fairly close fit on the rail.

Then I gave everything a coat of epoxy and the slide still moved freely.

However the first time I got a few grains of sand in the gap, it became almost impossible to move.

I did a fair bit of sanding to increase the clearance and it now works beautifully, even when it gets sand in it.

The moral is: don't try to be too clever and make the slider a precision fit on the rail.