he's how i do it.......
first decide on a position for the toolbox... not too low to get in the way of the endcaps or brake pedal operation ok

then mark the top and bottom on the leggies

next make a cardboard template that follows the lines you've drawn and the lines of the leggies
one for the top

and one for the bottom

then one for the sides measured between the 2 lines on the leggies

then use the templates to mark where your going to cut on your toolbox.. here you decide how deep or slim you want your toolbox
again mark the top

then the bottom

then the sideslining up the top and bottom with the lines you've drawn on the toolbox
