Bolting it to the table might not be a feasible solution
The hopping around is caused by the mass of the oscillating parts. Unfortunately, with your engine configuration, there's no way to properly balance it, but what you can try to reduce the vibration is to lighten things up as far as possible. For example, cut away any excess metal that's not needed on the big blocks oscillating between the pistons; from what I can see there's quite a bit of material there that's not needed, and adds considerably to the oscillating mass.
Kind regards, Arnold
