I would use the wrapping plastic from a household/food item, one that is stiff but easily bendable.
Draw and cut out the shape. Note, leave lots of plastic on all sides, so you won't spray on the miniature by mistake.
Then bend it carefully over the arm/location and spray on it. Wipe the plastic clean, and continue to the next one.
The issue with this method, vs. using a product made for this is that depending on the location you want the shape on, you will need to do a tiny tidying up on most models, i.e. tying up the lines, etc.
On the plus side, this is free, and it should be much easier than doing them by freehand. Though
@bOdziO Wolf has a point, if you do that many by freehand, you will get quite good at it
