A friend of ours gave us an old door that had been lying around in his grandfathers barn. It's a solid core door that has already been outdoors for years and has held up just fine.
Some of the styles were coming apart, so a few days before starting this project, we put extra wood glue in the joints and clamped them overnight.
Using the original hinges, we attached the door to the shed we standard exterior screws so that the door would hinge up. We used a few 2x4's to then support the door while we screwed eye bolts through the outside corners. Eye bolts on the shed were aligned to the eye bolts on the door - so when you fold the door up, the eyebolts are next to each other. This way, you can use a heave duty metal caribiner to secure the door in an upright position.
Medium strength chain was cut to length to allow
the door to sit perfectly level when unfolded.
Total cost of the project: $15 for chain and eye bolts