Hi Fran,
When you say a shade over 300 lbs., what is a "shade"?
If you would allow me, I'd like to take a stab at guessing your mono's weight from just a few simple deductions.
A 4x8 sheet of aluminum, depending on alloy, as far as I can tell without knowing the actual alloy, weighs on average 110-120 lbs. per sheet at the thickness you provide, (6mm/.250"). Or 3.4 - 3.75 lbs. per square foot.
So allowing for the benefit of the doubt, and without the roll bar that an RCR has to have, would mean that, using conservative figures, your mono has about 3 sheets of aluminum used in its construction, or 96 sq. ft.
Totaling 330 - 360 lbs.
The only thing I can't get my head around is how you only need 3, 4x8 sheets or 96 sq.ft. of aluminum to build a mono.
I have not included any of the extruded aluminum re-inforcements shown in pictures of the RCR mono.
Please help me to understand how an RCR tub is built.
Thanks,
Scott