Here are a couple of experiments along the same lines that I did a few years ago trying to save weight and money. In the end, I don't think I did either.
First- dollar store windshield reflector. Silver on one side, Speedy McQuuen Disney character on the other. Used alone at about 6,500 ft with a low of 36 degrees. Piled duff with the reflector on top. Did a fair job, but around 2 am it was cold and my hip bones were sore. Duff + reflector screen weighed more and took up more space than more effective solutions.
Second- Found a dollar store air mattress like you would use in a pool. Vinyl and bright pink but packed away nicely. I cut off the top section which was a pillow like structure. This gave me access to the air chambers. Cut 2.25" x 54" strips off a mylar blanket and slid them into the bottom of each air chamber. Resealed the end with heat and then glue as a second barrier. folds up to about the size of a coke can for a pad that was 22" x 56" x 2". This worked great in the places where the mylar stayed, but for about 2/3 of the pad the mylar just migrated or got balled up or otherwise wasn't where I wanted it. Where there was mylar it was almost to hot, where there wasn't it was too cold and I could never get comfortable.
Third- Using another dollar store air mattress and cutting it the same way, but this time I did two different things. I attached the mylar strips to similar strips of the thinnest climasheild insulation and very carefully spot glued the back of the climasheild before inserting. All but one chamber stayed in place. This was warm and cozy, but in the end weighed more and took up more space than similar solutions from brand name manufacturers.
Final solution, which worked as well as anything else but required a lot less work. The last dollar store mattress, uncut and still neon pink, with the Speedy McQueen windshield reflector on top and then me in a quilt. But doubling up the solution, reflector and air mattress was very space consuming so I ditched the whole effort.
I still like and use a cut-down version of the dollar store air mattress for warm weather. It, suprisingly, has survived about a dozen summer trips without deflating or popping. Doesn't offer much warmth and it is heavy by Ultralight standards (9-10 oz.)but folds flat and keeps me off the ground.