Okay, here's my testing jig:
The 10' long 2x6 weighs ~19+ pounds. The three wood rods are 1 3/8" (1.375") diameter (closet poles) and the two supports are 10" apart, with the top bearing rod is in the middle; 5" from the two supports. The center line of the bearing rod is 5 11/16" (5.6875") from the hinge. The 2' piece of the steel tape extends to 10' from the bearing rod center line.
When I put the digital bathroom scale (only accurate to about 0.5 pound) under the free end of the 2x6 arm, it correctly indicates ~9.5 pounds, because half the weight is held up by the hinge. When I put the scale under the bearing rod, it indicates between 155-160 pounds.
[Correction: This is where it gets a bit more complicated - the ~6" on either side of the pressure rod weighs approximately 2 pounds and it acts "normally", and the 17+ pounds (the weight of the rest of the bearing rod to the free end) are acting like they are ~9' away (the distance to the free end), which works out to about 155 pounds, which is almost exactly what my bathroom scale indicates...
That would be ~25.8 pounds per linear inch across the 6" wide sample. ] When I add my 25.5 pound cast iron/lead filled weight at 1', 2', 3' etc. along the 2x6 arm the jig will be exerting some serious force on the samples. The (estimated) 900 pound battery pack that is about 7' long will come to "only" about 10.7 pounds per inch - and that is across about 4' span - 8X more material, at least.
I looked for @#$%&+! hours today for my large C-clamps and while *know* they are somewhere (I last used them last summer...) I'm gonna' go buy some new ones to anchor the jig at the pivot end, so I can actually use it.
But in the trial run, the 10oz E-glass sample deflects a lot, but it holds up well and springs right back. I strongly suspect I will need at least 2 layers of glass fabric, and maybe more to get the stiffness I need.
I will try to dimension the pictures, if that will help clarify things.