I use what they used to call "red rosin" paper, it is available from your local home building center. I think they call it "builders paper" or "slip Sheet" it is a gray or brown color and available in the roofing section. It is stiff enough that you don't have to make the transfer to cardboard. What I do is get a piece fitted to one side by marking and cutting, then bring a piece in for the other side and do the same thing. When I am happy with the fit, while they are still in place on the floor tape them together with masking tape, then you have a full sized pattern. Hope this makes it clear....
I use it for making patterns for my vinyl too.
Aaron