: limitation on materials or structure. My question is 'what is the best
: strength/weight ratio for these given structure dimensions'.

I might ask an additional question: "given the simplest and quickest way to make a core, how much weaker will it be relative to the best I can do, sparing no effort" and "will the simple one be strong enough".

: I think your model is a different animal - the breakage would have to occur
: twice - on either end of the load.

I'm afraid I'll disagree here; I believe a core supported at the ends and loaded in the middle will break at the point load, in the middle. Why? because both ends will support one half of the load, while all of it pulls the mid point down. This will be a more practical way to do it. Also, with an extended roll, one hand pushes one blade up, the other pulls the middle down, and the lift of the other blade (not a point load, but anyway) pushes up also.

Not that it matters terribly, but just laying the core across two strong boxes, or chairs, or saw-horses, is much simpler. Less risk of splinters flying up when the shaft cracks, too.

Anyway, I'm sure you'll have fun doing those tests, however you set them up! Smashing things up... for a good cause!