I've had a few customers just bolt a box on top of the fender on the compacts. There is a lot more room on the utility tractors so I'm sure you may be able to come up with something better than the fender top. Should be plenty of room between the seat and the fender on the left side to fit one.
I'd look for an old surplus ammo can. They seem to be available in a lot of different sizes and are really well made with a lid that can easily be completely removed if needed. The lids also have a full gasket on them to keep your stuff nice and dry.
Your dealer can even get you a can of the right color spray paint if you want to blend it in or on the cheap, you can just leave it as-is or spray it black.
Ken
