After receiving some advice from my fellow YGSers, I researched pop up greenhouses a little bit.  For the price point and functionality, I settled on a FlowerHouse Pop-Up Greenhouse (3.5'x3'x3').  I am hoping that it won't get stolen out in my urban community garden...in fact, I specifically chose a small size that wouldn't be too useful for anything but gardening.  Anyway, once I remove a rather large Lilac from my garden, I'll have just the right space to put this in place.