I'm about to use a tar backed peel and stick, water shield and air barrier membrane from rona in my 142, the company name is Resisto. Looks pretty good, has a plastic top sheet and is waterproof. I plan on tuck taping seams as well, There's a web site on the box if you want to check out the product. Was way cheaper than dymamat, www.resisto.ca

I'm sure you can find something similar in the U.S.A. at lowes or home depot. It was 60-70$ canadian for 36"X40', more than enough to do my 142 floor pan and trunk if I remember correctly.