I have no data to back this up but my hunch is that fore an aft is just as important than side to side. I base this on watching riders ride these things. There is a constant small side to side and fore/aft action when someone is just steady state riding. But if you can add side to side cheaply, then that is still a win.
Motion rollers are another option for you. I found a set on FB Marketplace for super cheap.