Cut a piece of bubble wrap that is a couple inches narrower than the paper towel tube on either side. Glue one end of the bubble wrap centered on the paper towel roll. Wrap the rest of the bubble wrap ...

"But my bubbles are not pop-able - the paint inside is dry!" Up-close, they look like lots of bubbles of paint, but when a viewer stands back, they can see that all the tiny dots make up a masterpiece ...