If you go to just about any furniture store, you can ask (or dumpster dive) for the padded "bags" that sofas and love seats come shipped in.

I work at one, so they are easy for me to get, and I use them for everything from my bikes to my big bike to my lawn tractor.

The kind you want is the heavy plastic with a closed cell foam liner.

If it's outdoors, (I keep my stuff in the "barn" and it keeps the bird s**t off of them) use some old chain (swingset chain or heavier) and fold up and tape the chain into the bottom edge of the bag. If using indoors as a dust cover, omit the chain.

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If it ain't broke, and you mess with it long enough, it will be.