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Funny enough I've often contemplated making a similar device.
Although I haven't actually drawn up any plans, the principal should be easy enough to describe.
I presume when you say that the difficult is in finding a motor that you mean you can't find a motor with a lateral action, rather than a rotational one. If so, this method totally overcomes that problem.
The central section is a wheel (I would suggest a disc made from plywood or similar), about ˝ metre across, with optional holes drilled in radiating at even intervals from the central point.
These holes are to house the axles of the main piston, which is 'L' shaped. In this way, the wheel could be powered to a standard rotational rpm, but the frequency of the lateral piston modified by selecting which of the radial holes to insert it into. The inner holes would be shorter, but faster thrusts, while the outer ones would create longer & slower ones.
Needless to say, the FleshLight would have to be mounted in another side bracket. This should be even easier to make as it already has the threaded base that you could either adapt into a pivot, or use the thread to screw on a custom made pivot.