Here is what I did to my Kioti DK45. It is a water sealed Stanley FatMax 28" and holds all of the stuff I need. It is stuffed full... Stanley also makes an identically designed 23" wide box.
The first photo shows the steel supports for the box which are Stanley 1/4" flat mending plates and 12" angle available with pre-drilled 5/16" holes. These are available from your local home center. The front 2" of the tool box is supported on the factory cross beam with the rest of the box supported by the extra metal I added. So far it all sits level and rides well.
With the seat all of the way back, I can open the lid half way up and get almost anything I need. To open the lid all of the way vertical I need to move the seat forward some, but this is seldom necessary. The wrenches, sockets, ratchets, extensions, screw drivers and pliers are in two tool rolls. Everything else fends for themselves.
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