Buy a cheap rain suit and cut the sleeves off mid-fore arm and the legs mid-calf.
The non-breathable cheap rain suit will keep you warm and dry, and the cutoff sleeves and pant legs let in plenty of air and reduce the "sauna" effect.
Wear no socks. Wear shorts and a short sleeve jersey. Long sleeves and pant legs will get wet and "convect" the wetness to your body.
Dressing lightly under the rainsuit will reduce sweating.
Only ride as hard and fast as your clothing allows. In other words, don't build up a sweat.