You can use black rubber (or, more likely, synthetic) tubing and cut a slit in one side. The best way I've found to make a straight cut is to drill a hole just a bit larger than the tubing in a block of balsa and then stick a no. 11 blade through the block so the tip reaches a little bit into the hole. Then pull the tubing through the hole. The cut doesn't have to be perfectly straight. There are probably other and maybe better ways, but this will work.