For the past 6 years I've been using Large Nylon Laundry Bags, 30 inch x 40 inch from Amazon. Seller is Laundry Bags Online.
I use one bag per chair, and I also hang them from the rear ladder using a ladder chair mount carrier.
Slip a chair onto the rack, and from the bottom up I slip the bag onto the chair. The bag cinches up nice and tight. Keeps all the dirt/grime off and pretty much water resistant. Run about 10 bucks each in asst colors.
I carry 2 chairs on my rack, and after covering them with the bags, bungee them to the ladder by running bungee's top to bottom and side to side.
6 years now and have yet to have one "blown off". Still using same chairs that are also 6 years old. Bags also will slip over the larger "gravity" type chairs.
I recently took advantage of the companies lifetime warranty and they sent me two new bags after one seam started to unravel. Didn't even ask me what I was using them for, just sent me two new ones.
Steve & Cheryl + Zoey, and Ziggy, our furry kids.
2012 Forrest River Lexington 283ts
Toad, 2013 Subaru XV Crosstrek