Build the growing cylinder with wood post or with 4" pvc post.
Attach circular plywood plates at top and bottom.
(Optional: Cut half of the bottom out of a plastic pot and slide it down over the post, so that it sits on the bottom plywood circle. Put a few rocks in the pot to hold it down.)
Wrap insect screen around the plywood top and bottom plates and affix with staple gun and hot glue, so that a screen cylinder is made.
Cut a square hole out for a door, and cover with a square of screen that is 3" larger than the hole.
Use velcro or hot glue to attach flap, and you have a doorway to take plants in and out.