I like to use curved sticks to make mouthbows, but this one is curved because of the tension. It's made from a sliver of a splinter of a spruce tree that I found in a log jam in a local stream, and strung with 30 lb test monofillament fishing line. I find the smaller sizes don't have enough power and the larger sizes are dull and not so pretty sounding...
On one end the string is attached near the end and wrapped around the end of the stick.
On the other end, the line is wrapped around the end of the stick and tied with three loose hitches.
To tension the line, just put a bend in the stick, slide the hitches up or down and tighten them by pulling on the loose end... this is really quick and gives you a bow you can tune. You will find that at higher tensions there are less useful notes because the higher harmonics are out of the range of your hearing... best to use low tension.
You can get lots of monofillament in different diameters at a fishing supply store where they put new line on reels... they will have a huge box of old line... great for class projects or summer camp. If you're just on a fishing or camping trip and want to make a little musical instrument any size will work