DIY cold room ventilation
It really depends on what you want to store in it. If you are storing dry goods and don't need fresh air you would close of the damper in the summer. You can also control the temperature in the winter with a computer fan connected to a thermostat. If you are planning on curing meat, it's a lot more tricky.
Lowering the temperature in the summer depends on where the cold room is. If it's under the veranda keep it shaded and on extremely hot days soak it with water on top. As the water evaporates it will have a cooling effect on the concrete slab. It's effective and old Italian men sware by it. I've set up a couple passive meat curing cold rooms but it takes a lot of tinkering to keep a passive system within peramaters.
You could try this
I know a guy who ran the cold water from his water meter into his cold room based on me making a joke and he did it anyway and says it worked. He just hung a coil of copper and plumbed the cold in one side and out the other back to the original connection.
It went somewhat like this

Sorry for the crude drawing
I'm from Toronto so I know the temperature swings season to season all to well, and plus I'm an HVAC tech.