Cheap means silnylon, something like
LiteOutdoors Silnylon Tarp - 11.6 Oz. - Ultralight Backpacking Tarp or Hammock Tarp https://www.amazon.com/dp/B07WTZDX1C/
And polycro under your bag, not a bivy,
Duck 286218 Extra Large Patio Door Shrink Film Window insulation kit, 1, Crystal Clear https://www.amazon.com/dp/B000NHW2Z6/