A zipper with sticky strips on both sides is about 15$ I made a zipper door for my room and it worked great until the zipper broke.(prepare for yours to break aswell if you enter the room alot). Easy enough to replace though.
Use my door method, for it is the greatest!
4mil black/white poly
staples,screws,or roofing nails.
-cut 2 pieces to a size that would overlap past the door
-Lay both pieces over each other, and tape them together around all sides.
-Lay out the zipper in the middle of the poly, mark the height at the top and bottom.
-Cut a slit down the middle from mark to mark.
-Tuck tape both new edges(halfway for now) so they are sealed aswell
-Take insulation you've skinnied out so that its about r5. I used an r20 piece of insulation, stripped it into 4 skinny pieces and used that.
-Stuff your door on either side nicely.
-You need 'waffers' to hold the insulation up, so it doesn't sag over time. I cut up 8 small pieces of drywall 2"x2" and used 4 drywall screws to fasten each set together on either side of the poly.
-Finish taping off the middle poly when you've stuffed it.
-Add the zipper
-Tuck tape either side of the zippers adhesive strip(for added support and seal)
-use tuck tape and staples to hold it on the wall.
hang that whore and you're done! there you have it, 1 insulated 4mil poly door.
Lets hope that made sense, I'll go get a picture. Hurray for visual aids!