Use spray paint to make mask for pre-sensitised board?
You don't need transparent paper. You only need a mask that blocks the part where you want copper and passes UV where you want to etch the copper away.  If you use ordinary paper, it just means that you must use a significantly longer exposure time to properly expose the board.  And you must be sure that the black areas are sufficiently opaque to block the UV during the longer exposure.

But, no matter what you use, you should at least take a sample strip of the board and run an exposure test to see how long the exposure should be and what is the tolerance for exposure time, distance, brightness, etc. etc.

As @brhans suggested, you could also experiment with making the paper more transparent. Perhaps soaking in vegetable oil from the kitchen or even water(?)

Unless you have a very large and simple pattern, trying to cut out a "stencil" to spray paint the mask seems extraordinarily unlikely to me.