On my T1520, I mounted an A&I Products #A-TB11500 Plastic Tool Box behind the seat.
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It's not big enough for a hammer, but it has pins, channel locks, screw drivers, & gloves. I cut off the SMV bracket and installed an adhesive backed SMV sign to the back of the seat.
To mount the tool box, I have two pieces of 1/4" flat stock (1" wide, I think) that I attached to the seat belt attaching bolts on the cross bar behind the seat. The two flats extend back to support the box. I cut a piece of 10 gauge steel the same size as the tool box to use as a mounting plate under the tool box for reinforcement. The box is positioned as far back as possible to still allow access to the oil fill port.
I bought mine from Zoro.com, but it's easily available online from other places as well. My box has been on there for a couple years and a couple hundred hours and is still intact.
Late Addition: I did really want something a little bit longer, at least long enough to hold a hammer. But after searching, this was the largest box I could find that would not interfere with having a SMV, seat suspension travel, and access to the oil fill port. The box is not sealed, but I also usually have a rag in it and it usually stays clean enough for most field emergencies. If I ever find a longer box of the same height and depth, I will upgrade.