When I got the standard Jab cabin heat kit I didn't like the look of the butterfly. Just a metal on metal seat between the engine bay and the cockpit. So I welded a sleeve on it, and ran a NACA duct to the outside, so that when the heater is off, I can be sure no fumes can come in.
I now have an electric CO detector too. Apparently the first thing to go when you get CO poisoning is your colour vision. Didn't seem like a good plan to rely on seeing the colour change on the card type detectors. Not to mention the audible alert and flashing light I would now get, rather than picking the colour change up on a scan.
As for heel comfort, I would look at the shoes you use. Quality runners with a lot of heel padding might be the go.
I did 7 hours up to Bundy, and 8 hours back, and comfort was not an issue.
Except for the cold on the last 4 hours - I wouldn't have known if I had sore heels because I couldn't feel my feet at all.
I have since sealed a few air leaks so the cabin heat is more effective.