I started out today with a few goals for the hold down solution.
1) It had to be quick to get the jack on and off, with no tools. So, no bolts, wing nuts, loose hooks, etc.
2) Any parts had to store within the footprint for the base. For strorage, the base will just fit under the drawer in the back of the rig.
3) It must still function as a high-lift base - ya' know, just in case...

4) And of course, it had to be plenty strong.
So here is what I came up with. First, I used a little CAD (Cardboard Aided Design) to design and cut two pieces of 3/16" plate for the left and right sides of the jack. Since the jack is recessed flush with the base, they lay flat.
Nothing like a big band saw to cut pieces like this.
Next, I cut and tacked on two pieces which will drop down into the lattice on the base. These pieces will provide the mechanism for the hold-down.
I mentioned these magswitch magnets recently in another thread. Perfect for jobs like this. And even though they get covered in metal shavings when grinding around them, turn off the magnet and the metal dust and shavings drop right off. They are also crazy strong.
This is the left-side piece, tacked up.
It drops into the base like this:
I did the same for the right side:
Satisfied with the fit, I finish welded both pieces: