I bought myself a plastic, zip-up greenhouse. Forty dollars on Amazon and definitely worth every penny.
Five feet high with four shelves. You put something heavy on the bottom to keep it from being blown over in high winds.
We have no good place indoors for seedlings, and the growing season here isn't terribly long. Certainly nothing like what I see on the Australian blogs I read.
It has hard plastic pieces on the sides, with metal pipe to keep it all together, and a plastic cover.
The shelves are cheap and don't lay flat, but it was a simple matter to lash them to the piping.
They'll be fine. There isn't much in there now, but I have plans.