Yup, Ive done that. I built a DIY Pad with layers like this: ripstop - 1/16th CCF microfoam - space blanket - 1/16th CCF microfoam - fleece. I just made the ripstop an extra inch larger and used it to hem all the edges, then sewed lines across every 5", both ways (making boxes) to keep everything in place - and also to allow a tiny bit of water vapor pass through on the space blanket (you could also just run it through the sewing machine with no thread to make small pores in it to help with condensation).
It's 5'x3', and weighs in at 16oz. Not the lightest way to go for what it does. Still, it does work well.
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