This page shows how to attach the rectangular tenon part of the fork in the cheiroballistra into the slider.
Process step-by-step Edit
First drill a hole into the slider at the appropriate point. It should be slightly smaller in diameter than the rectangular tenon:
Next use a chisel to make the hole rectangular:
Optionally, chisel the bottom of the hole wider, so that the rectangular tenon can be riveted from below without it protruding from the slider and thus preventing it's movement in the case:
After this you can even out the rough edges using a file:
At this point round the lower edges of the tenon. This allows lowering (i.e. hammering) the tenon into the rectangular hole in the slider without damaging the slider.
Mark the edges of the fork part on the slider:
Now cut a small slot for the fork part. This makes the connection of the fork and the slider much more rigid:
Finally you should have something like this:
Once you're perfectly happy with the fork, you can rivet it to the slider from below.