Fans... Ice in water, etc. But most that successfully breed in a shed or garage have some sort of AC...I've heard that males can go sterile at temps over 80f, not sure how accurate that is.

It takes 21 days for females to give birth after being bred. Also, you will need separate tubs for mothers to birth and for pups to grow out. You have to separate male pups from their mothers at 4-5 weeks to prevent them breeding their mothers.

Personally, I'd also add hardware cloth to the sides...put more cutouts in. Rats can be susceptible to breathing issues and ammonia may still build up inside the tub with just the top screened.