So I didn't have cardboard tubes, but what I did have was plastic water bottles. We raided our recycling bin for water bottles with straight sides and found just enough. I cut out the tops with a utility knife, and brought them to class along with some clear packing tape.
During class, the children scooped out dried beans and peas, dried rice, and dried noodles to put in one bottle. Then we turned another bottle upside down on top of the first, and taped it in place. I think this version is even better because the water bottles have those nice grooves to help the contents rattle. Also, they are clear, so you can see what's inside making all the noise.
After that, we got out the foam stickers and decorated our rain sticks. By the end, my class sounded more like a monsoon than a gentle rain, but I guess maracas are good too. ;)