In the Bizarro world of MUSBJIM, where there's an easy non-complicated solution for most of your Speedster ills, I humbly (re)submit this solution. An $18 12-volt electric blanket (from Walmart) that you plug into the auxiliary power outlet of your car.
https://www.walmart.com/ip/Ele...mp;wl13=&veh=sem
Big enough to spread over you & your right-seaters' lap. Combined with wearing a nice cozy jacket & hat plus a wool scarf around your neck (plus the heated seats of your IM)...Voila, cozy top-down cruising!