I buy the cheap plastic painters drop cloths that they sell at home improvement stores. They are very large and work well when folded twice (4 sheets thick)
Years ago I used to line my trunk with one but since SUV's I simply line the cargo area. They are very cheap And are packaged very small to keep in trunk at all times. Buy a few and you will be surprised what uses you will find for them.
I have never had one leak and they fold up small so throw away is easy.

Mr.D