Don't know if any of you have tried this but Sauna's. Few years ago and our BIG ride of the year was going to be done in heat. Temps of 90f were promised so for a couple of weeks I went to the local gym and exercised in the sauna. First time and temp of 120f and very high humidity and 10 minutes gentle exercise was all I could take. 10 days later and it was 2 hours at a time- 3 litres of water and a weight loss of 3lbs every visit.

Didn't help much though- a Front came in and we had gentle rain all day long and a temp of 50F---In june.