Even just a tarp and some paracord would work. A few scrap timbers for a peak if needed and a big enough tarp so you'd have something to act as walls to keep the rain from blowing in. A trip to Wal-Mart should cover that. And you could still bring it in at night if you need to.
Not sure if this is your case but I tore down a little spot I had before I had materials to rebuild. Actually I was going to buy materials just was waiting on our state divs but the governor cut them in half that year. I settled for a homemade canopy with canvas tarps for forging for the last couple years. Which now I tore down and don't have another again (go figure) but I don't think I'll need one for awhile. Anyway its a simple fix and somewhat more ventilated than an indoor shop.
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