Obstruction Q - holding a runner on 3rd base

[JeffM]

I am a basketball official and parent of a 3rd baseman who is 12. He plays in a Cal Ripken league where they play on 70' bases and can take leads.

(I don't know why 3rd basemen at this age worry so much about the runner when most of the time they don't take a big secondary lead...)

When the third baseman is holding the runner on 3rd base, I know that both feet need to be in fair territory. Are there any other restrictions on where his feet are?

Is he allowed to straddle the bag so that his right foot is on the third base line? Can his right foot be on the edge of the bag closest to the plate so that the runner would need to avoid his foot if diving back into 3rd base.