I would recommend a high rise cap to give you a little more room to sit up in. You could go with a plain Aluminum cap and then line it with 1/2 foam to help hold the heat in and reduce condensation build up. Do the same below you with two pieces of 1/2" foam covered by a 2" Memory foam topper. Top it off with an extra sleeping bag on top. Next day clothes go between the first sleeping bag and second or in with you in the first.
Crack a window on each side and use a MyBuddy heater or similar, just make sure it has a low oxygen shut off. You should really only need it in the morning to knock the chill off.
My father and I slept in our truck once with an aluminum cap. Woke up to icicles 6" long hanging from the cap! We were cold, but not bad considering we only had a 40* sleeping bag and one wool blanket.