Someone up above said:

If you've never played a full night with the tub, bring an extra string and prepare for the worst.

When I got the first "starter cord" to build my super-tub I was concerned about wearing out strings, so I bought a 500 foot reel of cord for spares.

Before I used it enough to find out how long it would last, I discovered that the original cord (I think it was 3/8 inch diameter) was too heavy, so I went back and got a 500 foot reel of the lighter weight starter cord (about 5/16 inch diameter).

I put the first string on sometime, as I recall, in 1993. The same string is still on it, and it's been played by "anybody who wants to try it" at least yearly at the Walnut Valley Festival every year since, as well as at several other festivals and jams in Washington, Oregon, and Kansas.

At festivals I don't bother taking it in out of the rain, and it has sat unattended in the back yard through a couple of winters - and a summer or two, although when I could make room for it I've kept it in a leaky storage shed.

That cord is much to "strong" for a tension-modulated gutbucket, but mine is "fingered" and a heavy string helps get both tone and volume.

As mentioned quite a ways up above, I use an electric fence stretcher to tension it to around 60 lb tension, which would be tough to get with a broom handle.

The "clothesline" or window sash cord is approximately the same weight, but strength is lower and samples I've tried for other uses literally "come apart" at very low tension when wet, especially after you've worn off some of the surface "wax."

For a "stretch" gutbucket rather than a fingered one, parachute cord would likely give you the string weight you need for decent loudness/volume, and also has a bit of built-in stretch (to reduce opening shock on the tender parts) that would help get the pitch control. My starter cord intentionally has virtually no stretch.

(Your mileage is supposed to vary, because you're smart enough to make your design match what you use to build it.)

John