If you have enough fabric on your opening end you should be able to make the cut double the material on one side, set the zipper back 1 1/2 inches on that side of door and set the zipper right the edge of the other side. Then add one or two velcro patches, seam seal and test. That would appear to be quicker than searching for a water proof zipper if you are in a hurry to finish. And I agree with phat on the operation of those waterproof zippers - they can be really stiff a times.
Zip tech http://www.nrsweb.com/shop/product.asp?pfid=2288&deptid=1083
Works pretty good at helping waterproof zippers to ooperate. Actually I use it (it is like chapstick to apply) on jackets, pants, anything that has a zipper that is a little tough. One tube should last a lifetime.