Supported at an angle
The toolbox is between the Feifel filters and the mudguard. It is fixed at an angle.
The box is supported by metal bars, which can be flat or L-section or U-section (as indicated here). It's not clear to me why the box doesn't fall off. It is located over the inside half of the mudguard.
This damaged Tiger shows a supporting bar, which appears to be flat in this case.
Supported perpendicular to the wall
The track toolbox is held perpendicular to the hull wall, meaning that it has a tilt of 9 degrees.
It is supported by metal beams, as indicated here. There seems to be a central strap holding it in place. It is located over the center of the mudguard, close to the hull side.
Held in a metal frame
A metal frame with 2 straps is welded to the rear wall. It holds the toolbox perpendicular to the wall.