i had the same problem with my diamond plate toolbox. i upgraded to a steel one. prob 3x as heavy but iv had 4 people standing on the thing and nothing. im not sure on the brand though. i got a 5th wheel toolbox in my girlfriends dodge and i like it. even though im not a fan of them. its sturdy and works for us. i believe its a Husky and we got it used for 20 bucks off Craigslist. mine i got for free.

 

i have a better built...has a good lid, the inside has alot of support...never seen it sag at all