I used a Brute can for my mix tank and did not drill any holes in the can itself. All are in the lid only. The assembly photos below are of the DI storage tank but the holes are in the same places.
The tank on the left is the mix tank. I can roll it out to add the salt mix. once the water is ready I press a button and the controller will automatically transfer it to the center tank which is for fresh saltwater when it goes low. Once the water has been transferred the controller will automatically refill the mix tank with DI from the RODI. All I have to do is add the salt mix. The tank on the right is DI storage for ATO. Both the fresh salt water and DI tanks have pumps you can use to dispense either DI or fresh saltwater into containers at the tanks.