I think the source is not that great. I like MTM, but there really isn't a lot of reason to limit yourself to that brand.
Also, do think about how to mount. Unless you are very short and have the seat forward
, you'll have to add support since a larger toolbox will require offset toward the rear instead of centered. The stock toolbox on mine at least is only 4 1/2 inches from front to back.
On my L2501, this toolbox fits between the ROPS offsetted to the back. Not orange, but blends in well with the ROPS grey. It's 20 1/4 inch wide. I also modified the bracket to hold the slow moving vehicle sign that I cut off the attachment point from where it was mounted before. It has a removable top shelf. I carry relatively light items. Fwiw, the mounting point is very flexible, so having anything heavy in the box is probably not a great idea.
I'll create a thread with some more details with pictures of the install if there is interest.