1. Fasten together the 2×4 base with 3-inch screws.
2. Use a drill and auger to bore 1 ½-inch-diameter holes into the ground inside each corner of the 2×4 base. Bore two more holes, evenly spaced, along each long side of the 2×4 base.
3. Onto each end of the 1-inch-diamenter PVC pipes attach a reducing coupling and short section of 1 ¼-inch PVC pipe. Glue on the parts with PVC primer and cement.
4. Slip one end of the PVC pipe onto one of the galvanized pipes. Then bend the PVC and slip its other end onto the galvanized pipe protruding from the opposite corner.
5. Repeat the previous step to install the three remaining PVC pipes.
6. Secure the ends of each PVC pipe to the 2×4 base with U-shaped metal pipe brackets. Attach the brackets with 1 ¼-inch screws and washers.
7. Build two doorframes out of 2x4s, then screw one to each end of the 2×4 base.
8. Cut diagonal 2×4 braces and install one to each side of both doorframes.
9. Next, cut two long 2×4 beams to span the length of the greenhouse. Screw the ends of the beams to the doorframes.
10. Build two doors out of pressure-treated 2x2s.
11. Set the doors in the doorframes and attach each one with three 3-inch butt hinges.
12. Screw a short wood block to the doorframe to act as a latch.
13. Cover the entire PVC-pipe frame with 4-mil plastic sheeting.
14. Secure the plastic sheeting to the 2x4s with ¼-inch staples, spaced 3 inches apart.
15. Also, staple the plastic to the doorframes and to each door.
16. Use a utility knife to cut the plastic sheeting around each door.