A clear plastic roof may be just what you need. You can get clear corrugated roofing at your local building supply store or try something a little different. The material in this photo is inch thick honey comb plastic from Tap Plastic.
Whole rows of plants can be covered using long, low tunnel cloches, which are left open at the ends for thorough ventilation or closed off when greater protection is required. No rain will reach cloched plants, so remember to water them as necessary.