I too found a piece of thick painters tarp to be adequate.

I found that if I put in INSIDE the tent under my bag, it was a fine vapor barrier but if you put it under the tent it funneled water under and you get MORE WET. We have demonstrated this often, try it yourself if you wanna get wet put the barrier under the tent. Space blankets are a waste. I want gear that can be used over and over.
Jim
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.