OOPS. You'd have to have a bottom on that toilet paper roll. But actually if the dog spray container had a little piece on it to slide over the edge of the toilet paper roll, you wouldn't want a bottom - in case it rained. You wouldn't want it to get full of water.

Or you could get a pvc pipe that's a little bigger in diameter than the spray can, drill 2 sets of parallel holes in it, then snake zip ties through and around your stem. OR if you don't want to scratch your stem with zip ties, go to a craft store and buy strips of velcro that you could cut, slide through the holes on the pvc pipe and wrap that around the stem.

Who knows? Maybe there is a product already invented just for this purpose that I don't know about.