I had this EXACT idea earlier. Even down to the different ball sizes and "locking" the balls in. (Although I didn't call it that) I think the only differences were I had a shorter seperating wall, only 2 ball sizes instead of 3, and no footballs
Anyway, I won't be competeing, but I made an idea for a design anyway
Assuming its legal to cover your own triangles, I would make a bot that could cover both triangles and has a massive hopper in the middle that your allies can fill up, that dumps all the balls at the last second.