Captain Antilles said:
But something seems off about the waist area of that 3PO costume (though the rest of it looks excellent). Too long? Anyone else notice?
It's just the costume slipped down a bit by the looks of things. Here are some other photos of Simon in the costume, and you can see there is nothing wrong with the length of the waist area
plus the original 3p0 costumes waist altered length all the time throughout the films.