I would put plastic containers of water under your seats, but remember you have a limit of 50lbs per seat. Also don't put full fuel in the machine, whirly is quite right in stating the main tank only will help. I WOULD NOT put anything on the floor as it could slide around and the weight is meant to be taken on the seats. You could easily end up out of CG if it slipped to far forward.
With regard to what you can put under your seat VeeAny is quite right with regard to it being collapsable. I've just completed the Robinson Safety Course and the first thing Tim Tucker did was to check under his seat to see what would go up his **** in the event of a hard landing. He also showed a video of an R22 that had a hard landing. The Heli looked ok apart from the splayed skids.....the seats looked perfect! Both occupents died from internal injuries as a result of the landing and on examination the seats were full of paperwork and survival equipment which simply didn't compress. The opposite of this happened at Newcastle a few years ago when an R22 crashed and the guy walked away, Heli looked totally wrecked, so beware!
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