What's the capacity of the coolant thermos?

Just ruminating on the possibility of retrofitting one or more of these tanks to a GMC truck that has a lot of empty space under the hood.

Plumbing them into the coolant system with valves that open when the engine is running then close when it's off would be simpler than how a Prius uses it. Start the truck and it'd get a shot of hot/warm coolant to warm up quicker. With coolant constantly circulating to the thermos tank(s) there would be no need for a fancy pump and complex computer control. For the valves I'd use a design that is either held open by vacuum from the engine, or uses a motor or solenoid to switch states without needing constant power to hold open against a closing spring. Electric valves would need a simple circuit connected to the battery with a way to sense when the engine is running or not so they could close after the key is turned off, and wouldn't open with the key in the accessory position.

Makes me wonder why Toyota didn't do it in such a simple way? Then it wouldn't matter if the Prius sat long enough for the thermos to cool off. It'd just open and get heated up when the engine starts, but not when running on just electric.