I don't think I would do that. If the side panel is flexible at all there is too much danger of jostling them while they are running. Best to get them inside the case if possible. Sometimes you can steal a drive cage from an old case and modify it or fix to the inside of your case. I used to have several of these lying around but they disappeared in the recent spring cleaning. See if there is a used computer dealer or repair shop in your area that might have one for you.
....WW (5.1)