Hi Jerry,
I don't know that anything like that exists specifically for stage managers. However, there's nothing stopping you from using a template made for another purpose- I have one that uses basic shapes. (I mostly just use it as a straight edge when penciling cues into my book.)
Or, you could make it yourself. I know one stage manager who made rubber stamps- she stamped her template on, then wrote her cues in. You could do that, or take a cereal box or stiff piece of plastic and an exacto knife and cut a template out that fits your needs. In my opinion, it's probably easier to find the template that has the shape you want in the size you want, and then just use the one you need.
Draftingsteals.com has a ton of templates. (They even have ones with little furniture, like pianos and couches, if you're inclined to use templates for blocking.)
Best of luck!
Maribeth