Someone on here with a P5 used some 8" or 6" diameter plastic pipe with a cap glued on one end and screw cap end on the other, like large water main pipe. Strapped it to the outer frame under driver's seat. Gonna do something like this too. Very strong water proof and easy to do. I did give consideration to slide drawer runners and bolt an ammo box to them and bolt to frame under the driver's seat. Then place a door such that you would open it and slide the ammo can out to the side of the machine. Just not made time to do either.... yet.
Put an ammo can under the hood for second battery and a small bit of storage for manual, insurance papers etc.
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