As mentioned above, add wheels, or put a 6" piece of 2" receiver stock vertically on each corner, so you can slip in the wheels when you need to, and take them off when you don't need them.
Add a 2" receiver out the back side to attach a trailer ball, a towing hook, a small step platform, a vice, a winch, a small boom pole, a quick add-on small rake.
Fill the lower 2/3rds of the box with concrete. Leave the top third open to carry the chainsaw, bar oil, gasoline, gloves, chaps, hardhat, axe, lunch, water jug, hammer, extra lynch pins, chains, trailer ball, a towing hook, a small step platform, a vice, a winch. Make a lid to close down over it all to keep it dry, and safe when you lock it up while the tractor's outside.
Add a couple 3 foot pieces of 3" Shedule 80 iron pipe vertically. Close the bottom with mesh. Use them to carry shovels, rakes, sledgehammers, big persuader pry bar. If you make it with 4" pipe, I bet you could store a lot of chain in a pretty small footprint, and you can store your small boom pole vertically also.
Weld a piece of angle iron off the side so you can scrape your boots when they really get caked with mud.
Put a bracket on to add suitcase weights for those times gravity needs some help.
Weld a quick-tach plate to the box so you can use it with your quick-tach FEL, for the times you need counterweight for heavy 3-pt implements.