I would suggest using ordinary rags or a old bath towel. Plastic is going to trap moisture and cause corrosion if you seal it too well. The towel or rags will wick moisture away and allow it to evaporate while keeping foreign objects out. If you have it outside without a carport or garage over it you are asking for trouble no matter what you do. If the hood is over it keeping wind and rain from directly getting on it it will be ok as long as you seal the edges of the towel or rags to keep the wind from blowing stuff up under them. I built aircraft turbine engines for 8 years. I have experience with contamination and moisture prevention