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waiting for a company to make neoprene ones with velcro covers for us.
I think it is about cooling the units, neoprene is a great insulator...
How would that be an issue when the drill is just sitting there? Surely your not thinking that any air cooled motor (gas or electric) should be used with the cover on?
I used to frame 14ga steel with my Makita impactor outside in the rain and no problems.