I would think some sort of steel would be in order. I think I am picturing your situation, and if it is what I think it is, it would seem you are putting an extreme amount of weight on the hatch. I am not sure what your round bales weigh, but mine are around 600 pounds, then being carried way out front of the tractor, that is a lot of weight on that front axle (assuming by spearing you meant with a front loader). I move my round bales multiple ways:
Bulldozer with the bale chained to the dozer blade
Log loader (the fastest and best way)
Using my winch on my farm tractor so the weight is on the back
Strength to weight wise, I would think steel would be your only real
answer. Finding something that would work at the junk
yard would be my best bet. On my farm I scrounge every pound of steel I can get, I just never know when I will need it for something.