Have you considered tractor weights? They make front weights and wheel weights to add mass to the front of tractors. Wheels weights are typically donut-shaped.
ISteel, not concrete, so denser and won’t crumble under stress.
Example:
tractor weights sold by the pair $150
http://dallas.craigslist.org/ftw/grd/5796126424.html
We use plow discs for blast deflectors for high power rockets.
http://www.agrisupply.com/plain-disc-harrow-blades/c/4500011/
They nest nicely, so if you stacked them, they may do what you want. I cannot tell you whether they are available on the used market as we have had ours for a very long time.
That’s a good suggestion, but $0.75/pound is still much more expensive than $0.05/pound for concrete when we need 1,000 pounds. I have spent around ~$1,150 of our $1,400 budget so far, and we still need to purchase the concrete, rebar, paint
Could you cut thin plexi down to strips and join the ends to make the form?
In regards to tensile stiffness, they make fiberglass reinforcement you could add
https://www.amazon.com/gp/aw/d/B000CODWAE/ref=mp_s_a_1_1?ie=UTF8&qid=1474925158&sr=8-1&pi=SX200_QL40&keywords=concrete+fiber+additive&dpPl=1&dpID=5170K9ENFWL&ref=plSrch
Tractor weights, unlike concrete discs, have resale value at the end of the project. You could buy them, use them, then resell them when dismantling the trebuchet for likely the same price. Net cost: $0.00/lb