Check the radar. If there is a line of storms coming , DON'T SET UP!
I'm dead serious. I have played hundreds of festivals. And I have canceled dozens. I show the radar to the event coordinator. If the storms are clearly on the way, I'm not setting up. Period. It's both dangerous and impractical.
We also had a policy (in our WRITTEN CONTRACT) of getting paid in full if we showed up (and of course we already had our deposit). If the weather was looking ominous, we would call our contact on the way and remind them that if we show up, we get paid in full. So they had our drive time to make the call.
That being said, the cheapest and most compact way of getting it done is the thicker painter's "drop cloth". There's nothing "cloth" about it. It's plastic. Get the thicker mill stuff. It will come in a roll in a box similar to aluminum foil or wax paper. It unfolds after you pull off the length you need. Drape it over your rig and tuck it under the feet. A good stiff bungee cord around near the bottom to hold it just in case. Throw it a away when the storm passes and roll out some more next time. It's much more water proof than most tarps.
This would last you the rest of your life. I'm sure there are cheaper options out there.
HDX 10 ft. x 100 ft. Clear 6 mil. Plastic Sheeting CFHD0610C at The Home Depot - Mobile
Also, I have the "roof" of an old tailgating tent folded up in the back of my truck. The frame got broken and rusted so I kept the top. It works great in an emergency for anything, not just bass gear. So if you have an old one with a busted frame tucked a away in your garage, yank the top off and throw away the frame.