Needed: Ideas for a cheap temporary Floor work surface

I'm trying to find a very cheap yet safe floor surface to work on my cars on. It needs to be able to support the weight of a car on jack stands and a floor jack etc.

I know that the best way is reinforced concrete. The problem is I rent this house and I know that I will not be reimbursed for any modifications I make.

What do you all think? I'm looking for the least expensive way possible.

Thanks in advance for your suggestions.

C