I bought this side tray for my 44” US General lower toolbox and drilled holes to support ratchet extensions. Fits well with the towel holder, too. Very strong padded magnets.
There are a lot of great ideas for making modifications and add-ons to tool carts on the "Modifications to the HF 4 and 5 drawer service carts - what changes have you made?" thread. You should post there too, your mod will fit right in!
Fun fact: I stole the idea from someone on the internet...
I carry one of these HF trays in the camping SUV and when you get the camping chair out you stick the tray to the side of the truck to hold your drink/phone/ or whatever...that way it's not in the dirt or getting knocked over by dogs. Perfect for short breaks when you're not breaking out full camp.
I think it is a security issue. Most guys with tons of tools want to lock them up. Either at work (to keep co-workers from "borrowing") or at home to keep the kids out and the garage tidy.
The manufacturers then have a limited customer base for outside the toolbox storage needs.