One common objection to open-cockpit airplanes is that one must stop flying in the winter. Both Moonraker and N500F were well-sealed forward, which helped keep me warmer. I've flown in the front seat of a Stearman on a 40-degree day. The wind blast through the cockpit made the ride more uncomfortable than my 25-degree ride last weekend. If you build a two-seater like a Pietenpol or Murphy Renegade, build a cover for the front pit to cut down the drafts when flying solo.
Heaters help, even in open cockpit planes.
At no time last Saturday did I feel warm air hitting my legs from the full-on heater. But at one point, I pulled my glove off and stuck my hand down into the tunnel. But keep in mind that unless the drafts are elminated, you lose a lot of heater efficiency. Proper clothing is a must, but don't go overboard. I have a friend who dons a snowmobile suit for all flying below 40 degrees.