Before we go any further can I try and dispel the myth that a tea chest bass is only used for Skiffle,I have been playing mine (plural) for many years now and can only remember a couple of occasions when I have indulged in the Skiffle Art. Mostly I play along with any music that is being played. Blues is especially enjoyable. I have modified my basses with a plastic ring that slides up and down the neck thus shortening the string. This is not to change the tune but more to set the range, something like a trombonist I think.
I hope it isn't necessary to make your own there is something special about playing the real thing, Although I did replace the bottom/top of one of mine when it filled with rain!
You may have seen the other mod I have made by adding wheels and converting to a trolley.
Andy