I have tried several types and sizes of latex tubing (sold as surgical tubing, but also for scuba diving and other purposes). I suggest starting with a 3/8″ inner diameter (ID) with a 1/8″ thick wall if you can get it at a reasonable price (in a quantity of a few yards). A smaller inner diameter can be stretched to fit but does not work in that it is too easily clogged with moisture and starts gurgling. I found black 3/8″ ID tubing that works well except that it has a thinner wall that too easily kinks like a bad garden hose. The thickness of the wall may also have something to do with the gurgling problem. A thinner walled, more flexible tubing will also expand and contract when you’re playing, creating slight delays. If there’s a medical supplies store or a scuba gear shop in your vicinity, there’s a good chance they could help find the right tubing and sell you the length you want. The 3/8″ black tubing works nicely for a short run of about 5-8 inches.