This is one way to build a simple tri-string bubbling rig, using easy-to-find materials.
2 4-foot, 3/8 inch poplar dowels 137 inches of 100% cotton cooking twine 2 1-inch wooden dowel caps with 3/8 inch holes 1 15/16 inch split ring
Here in the U.S., the best cooking twine can be found at Bed Bath & Beyond. I found the dowel caps and split rings at Hobby Lobby. The dowels from my local Menards are generally less crooked than Home Depot's (not that it matters much).
1. Measure and cut a piece of twine 137 inches in length.
2. Wet the twine with water, and pull the length of it through a thumb and forefinger. Hold your arms up high and keep the ends apart as you do this, so that the ends can dangle freely above the floor as much as possible. You'll see each end spin quite a bit as you approach it. Repeat this, back and forth, until the spinning stops. 4 or 5 times is usually enough.
3. Wet the twine again, and hold the 2 ends up and apart so that the twine forms a "U" shape above the floor. As you bring the ends together, the "U" will typically twist up on itself, forming a "Y". Twist one of the ends clockwise (or counter-) to remove the twist, so that you can bring the ends very near and the "U" doesn't twist.
4. Tie the ends together with a square (reef) knot. Pull it tight and cut off the extra ends.
5. Form a loop end in the twine about 4 inches from the 1st square knot, measure 8 inches from that loop end, and tie a simple overhand knot at that point (in the doubled-over twine loop).
6. Measure 41 inches from that 1st overhand knot, and form another loop end there. Again, measure 8 inches from the loop's end and tie an overhand knot at that spot, in the doubled twine, as in the previous step.
7. Place one of the loop ends on top of a dowel rod's end, and fit a dowel cap onto the dowel's end so as to secure the twine. Twist the cap as you push it on. If the fit is too tight, shave a bit of wood from the edges of the dowel's end. If the connection seems too loose, place the loop end across and over the dowel's end. Repeat for the other dowel, loop end, and cap.
8. Attach a split ring to the longer section of string. (Optional).