Yes if that gear is strong enough to make a cap out of it would be a cheap alternative and easy to work with. Although, $27 plus something like $7 shipping is not very expensive to get a complete LST XXL diff. All you need after that are the Traxxas shafts and a Traxxas output gear which you still need even with that plastic Kyosho gear.
hahaha waddup guys, only thing that I would do different with the plastic gear is use 4 posts instead of two and I know you cant see it in my pictures but there is actually four posts there.