I am looking for a tool box/chest with a 300*400mm footprint to fit on the Tern Transporteur front rack
How secure does it have to be?
Will it have to detach quickly?
Does it have to be light?
Stanley do a lot of sizes in their Fatmax (plastic) range.
Milwaukee Packout (small) is about 250x410 if that's near enough?
https://www.milwaukeetool.com/Products/Storage-Solutions/PACKOUT/PACKOUT-Tool-Boxes/48-22-8422
400 x 300 Is a standard 'euro box' size so might open up a few options - does it have to be a tool box/chest or could it be a lidded container?
As above that's Eurobox profile. Laura Kampf has done some videos using that form factor.
Eg.
You can get them with lids, Bigdug would be my first port of call if you can't find any second hand. In fact is Festool that profile perhaps?
That's Systainer size footprint.
It's an interlocking box system sold by Systainer but also used by Makita, Festool, Panasonic and other power tool makers. You should be able to get a selection of empty boxes, drawers and organiser inserts to suit your needs, there's always a ton on ebay. Not cheap, but super robust. You'll also be able to get clips to put into the base of your rack so the whole thing just clicks in and out as needed. Like this:
