I would think canvas too stiff for a very good cover. I have bunch of old sheets that I use for drop sheets or covers. They drape loose fit well around the contours.
For moisture control, the best way is to keep the shop from getting too cold, so water doesn't condense on the iron when it warms up.
My jointer used to rust like crazy until I noticed and where some plywood was stacked on one end, it wasn't rusty under it. I cut some MDF to fit the bed and back fence and rusting is a thing of the past n that machine. It would work on any unprotected flat surfaces, I would speculate.
I have several HTC machine covers that I got at Rockler. They are coated and supposed to breath without letting moisture in. They are a bit expensive and to be honest, they are kinda stiff and don't cover the machines tight. I would think an old bed sheet would work just as well.