Simple fix for this. Take a piece of insulation foam (go for half inch thick), and mount the hard drive bottom to it. Then, take the same foam and cut blocks about 1 inch square out of the same type of foam, mounting them to the bottom of the foam on the hard drive. Another full sheet on the bottom of the hard disk and it'll be good to go, just attach the very bottom piece somewhere on the comp case.

I'm pretty sure this'll work, but not 100%. The vibrations of a lawn mower on the ground may be a little more than a motor boat engine a friend of mine modified. The foam should dampen most of the shock to the HD, but you might want to increase the thickness of the blocks, to give it more room to move.