An update......
The "jumping jack" tamper thingy was a total bust -- at least for our purposes. And it probably has a lot to do with our berm-prep set-up [and maybe our expectations....?], though I don't have enough knowledge about these things to say just what went wrong or what would have made it turn out better, if anything.
Rented one to use over the weekend. We were shown how to use it -- basic operation is fairly straightforward.
But when we set it up on the berm and started it up, it pretty much destroyed the berm as well as the forms we had to confine the dirt. You might say that it was over-kill.
It
did compact, but it did it in such a way that the dirt went out laterally, in all directions, despite our walls. So disappointed, as it really seemed like it was going to do the job for us. Maybe if they made a smaller version.....but that probably wouldn't be practical from a construction standpoint, so.......it's back to hand-tamping!
This would be perfect -- a powered version of the manual tamper. I going to call every rental place in town tomorrow and see if ANYONE has one.....