OH--I had a bb just like the silver one with the gold long horn steer on it when I was in college.
I don't see Ennis wearing any metal on a belt except the buckel---you don't want all that metal around your waist if you are going to wear it on a daily basis---first it gets too dirty, and second--if he ever hits the ground off a horse--it pokes you and hurts. That is also why it can't be too huge, and in my own opinion---oval would be the best shape---square or rectangles have sharp edges, unless they are small in line with the leather--(used to have a big square one, and first time I got pitched off--it left my belt QUICK!!
IF he is going to only wear it around Ellery, when they go out and such, whatever is okay--but if this token BB is going to be something he wants to wear on a daily basis to remind him of Ellery--I see it being inlaid and not too many stones or upright projections.
There are a lot of neat choices here already. AND since it is going to be worn next to leather, and clothing colors and NOT Ennis' skin---the color does not matter. the Gold and or silver--that are a person's best colors are only valid next to skin---NO Matter what color the buckels ends up--It will be perfect because it is too far away from his eyes or face to matter or make a difference.