I looked up the retail price of just the parts to build that box and it totals $161 + shipping costs (assume $25) and time to do the ordering (assume your time is not worth anything) for a total of $186. The cost of the PowerWerx box is $169.99 + $35.23 shipping for a total of $205.22. So, if you have all the things necessary to machine the box and make good, crimped connections and you assume your assembly time and labor are not worth anything, you would save $19.22.
Just the box is available on Amazon - search for: MTM Ammo Can - Dry Storage Emergency Marine Box - AC35