Just in case this is a useful suggestion: try checking out auto racing suppliers. There are undershirts with tubing networks for cool water. They're meant to go

Just in case this is a useful suggestion: try checking out auto racing suppliers. There are undershirts with tubing networks for cool water. They're meant to go under the fire resistant suit you have to wear under more serious racing's safety rules. The rigs I've seen have a reservoir with an ice pack and the equivalent of an aquarium pump that you put in a mini cooler bolted behind the driver's seat. You could probably macgyver up something similar with a small hydration bladder style backpack that might give you a few hours of substantial cooling on the go.