What about using HDPE for the uprights?

I know its not entirely comparable but with a glancing blow NST shattered my 10mm Nylon base on conker. The only reason it didn't fall apart was a single screw in the middle of the machine.

The rest of the armour was HDPE and it stood up to the hits far better as it could bend and flex a little.

Its looking very promising though! I am more worried about that now than I am about 720!