I remember making a washtub bass more than 40 years ago. I used an actual double bass string, LaBella I think. It was a white colored wound string, nylon as I remember. I can't remember which string it was, probably A but I just don't remember. It worked great with good volume and tone production. The broom handle was just however long replacement broom handles are at the hardware store. I think you'll want to fasten the string so it goes over a notch in the top of the broomstick before heading down to the washtub. That way you can tension it using the broomstick alone or by sliding your noting hand down the broomstick to produce the higher tones.

I too have heard these played right in tune.

      - Mark