Holding the door open is the most practical.
I've used the bandanas with the gel in them that hold water; you can find them at home depot or walmart. I sent a case of them to my son when his unit was deployed in the middle east. The idea is that you soak them, they absorb water, and it cools you as it evaporates.
Having spent a ridiculous amount of hours and years in greenhouse canopy cockpits that don't cool well or cool at all, in the middle of summer, with helmet, flight suit, boots, gloves and gear on, I learned to carry a rag to wipe sweat out of my eyes, and to carry plenty of extra water and gatoraid. Hydrate more than you think and force yourself to drink water.
I've gone eight hours in the cockpit without ever needing to urinate, while drinking water continuously, and ended the day with a flight suit white with salt stains from sweat. Even forcing water it's evident I wasn't getting enough: you'll need more than you think at times.
If you're just going out for an hour or two for flight training, pick the cooler parts of the day, when it's calmer anyway, and enjoy those. Night flight is relaxing and often quieter with less traffic, which means that you can sometimes get more done.