Rain, rain, go away

Greenhouse constructed of PVC pipe and plastic film

We have an 'event' coming up and now it looks like rain. I'm thinking that we should have a backup plan, you know, just in case our luck should turn sour and it does rain. It's not going to rain, but you know, just in case.
    So I'm thinking a giant tent made out of cheap plastic materials would be just the ticket. Looking around I found this picture on Simplifed Building, who seem to have some good ideas.
    As to whether I actually build such a structure or not is still up in the air.  It would run about $500 for materials and cover an area about 700 square feet. But when do you set it up? It would take at least a day to set it up, so it would have to be set up the day before. If I am seriously considering this I should go buy the materials tomorrow, just I have them on hand. Urk. $500. If I don't use them, I should be able to return them. Still, kind of a big commitment. And if it does rain, would something like this suffice? Fret, worry and concern seem to be mine.