After purchasing a much needed patio sofa set, I had a dilemma with how to store those large cushions when not in use. There was not a plastic container large enough to fit them at the big box stores and purchasing a large storage container that might double as a bench at 1/3 of the cost of the patio set was out of question.
The solution was zippered garment bags that I found at a big box store for $2.97 - I needed four. Two cushions fit perfectly and they zipped up to keep pests out. I added cedar disks for additional pest control and to keep the cushions smelling fresh.
This page contains the following solutions.
Want to leave those outside chair cushions outside but keep them protected? Make a "pillowcase" from heavy MM clear plastic, cut slits where the ties of your cushions are, bring the ties out through the plastic, and tie as you would secure your cushions.