I've got a mate with a side mount on a cyma, Works very well.
When he installed it he knew the metal on Cymas is pretty craptastic, So it's not only screwed on, he took the time to epoxy it in place as well. you'd think it was a part of the original cast now.
In operation it takes seconds to put the scope on, So it can be removed very quickly for battery changes without any tools, or messing about.