I thought I would share some pics of a vibrator I cast in aluminum for use in my small foundry. I followed a design in 'Metal Casting: A Sand Casting Manual For the Small Foundry Vol. 1', by Stephen D. Chastain. I made some slight changes to the base, compared to his design. He made the vibrator to help him release board mounted patterns from the sand after ramming a sand mold.
Compressed air drives a ball bearing around in a circle and escapes out of small holes in the top of the casing. A piece of thick walled steel tube is cast into the part to make sure that the casting does not crack and explode under the pressure of the ball spinning. I imaginge that these could be cast in e/g instead of aluminum. You could cast four of these in one go, easily. The force of a 5/8" ball is not very great, so I would make the ball bigger. by changing the air pressure you can change the force and frequency of the vibration. You might be able to get a higher frequency than the $40 concrete vibrator. I am sure you won't match the force, though. But maybe frequency is more important than force. And the direction of vibration is horizontal with the air vibrator, wheras the electric one seems to vibrate on the vertical plane. This could make a difference in compacting and de-airing the e/g mixture, who knows? Might be worth a try.
Dave