Sides sides sides. At least be able to cover two sides completely, which should give you the ability to block the rain where ever it decides to come from. You don't have to buy fancy, expensive, made for whatever sides. The hardware store will have normal tarps that will fit pretty close and you just fold up the bottom, or roll up the end to set it to whatever length you need. Always fold inside so it doesn't catch the rain
. For holding them on, ball bungies are nice, but more of those clamps shown above are quicker to put on and effective. If your hardware store doesn't have the tarps you want, check tarpsplus.com, they have a ton of lengths, widths, weights and colors.
Neil -- "Easy Bake Master"
NB Bandera [gone to heaven...conveniently located in Long Island, NY]
Rebel 23, WSM A, WSM B, UDS, Easy Bake Oven
Pork Pistols BBQ
MOINK...It's what's for linner.