Up for sale are Snap-On top, roller and side tool boxes. The side boxes have a lip at either side where they sit onto the main roller tool box. Screws can be used(not supplied) to affix them securely to the main box, but I never done this and they've been fine.
Here for sale I have my Snap On Tools 54" roll cab in black with a stainless steel work top. The price is £1750 and includes a key (only came with 1), a handle,all drawer liners, a side shelf with a rubber liner and a top rubber liner which is handy to protect the stainless worktop.