Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
killing them will only satisfy your anger...There are way too many to battle. you don't need traps, you need a deterrent like moth balls, peppermint oil, dryer sheets, barn owl... Start with plugging all the holes and access points. Once the infestation settles down, then move on to traps. This is also a great excuse to enter the world of PCP air guns...If this isn't possible, then move. All you need is rat poop in a box of finished product ...health code violation for employees..sounds troublesome
Get this stuff.We use it for bee proofing. Nothing wants to eat through copper mesh. Lolhttps://www.amazon.com/Stuf-Fit-Copper-Mesh-Birds-Control/dp/B0001IMLTY/ref=sr_1_1?ie=UTF8&qid=1508444450&sr=8-1&keywords=stuff+it+copper+mesh
that's cool. They aren't stupid animals. just gotta build a better mousetrap.You will have to really seal the entry points. I've heard they don't like to chew great stuff foam, but I doubt that is true. You will need to use metal flashing to seal the entry points.
We have a peanut/cotton field right behind our shop and ever now and then we get rats in the shop, couple of traps, we good to go.