I like both those plans. You might try using the reflective safety sheets used to help people stranded in cars. We got some for our window shelf perches. I put them over the cushions and then put a blanket over top. Very economical and work really well. Here is what we got on Amazon for $15.00 "72 HRS MIL-SPEC Emergency Space Blankets Mylar Survival or Emergency Thermal Blankets for Camping, Hiking, Marathon, First Aid, Emergency Preparedness, Extreme Weather, Shelter "

for your outer insulation, you might consider getting something like this "REFLECTIX BP24010 24"x10' Foil Insulation "