Reversible Belt Buckles
Reversible metal belt buckles! You can wear it pony side, or be a bit more stealthy and swap it to the cutie-mark side instead! They're nice, solid metal, and the top part can rotate when you put the belt on, but will stay nice and solid once it's on.
The belts they fit are 1 3/8" wide, though you could use a 1 1/2" belt as well, it'd just take a bit of wear. I use this one here and it works quite well!