>>1414653People used wool blankets in the past. To reach about R5 you need about 2 inches of wool blanket. That's like $20-$30 from Harbor Freight:
https://www.harborfreight.com/60-inch-x-80-inch-wool-blanket-92625.htmlSheep's wool is about R3.7 per inch, fyi. So, you'd need probably 3 of those blankets to get that much. They are only $10ish each. I have 2 of them for my vehicles in case of a break down.
If you are DIY minded, you can make a $30ish R5 sleeping pad yourself from these materials in this image. The up side is that it can be 4 feet wide instead of being really narrow like the expensive ones. The 1-inch thick foam have an R-value of 5. It will also be a lot less weight than the wool, but cost about the same.
