Called a tank probe. It is what most diggers use. The length is what you have to think about. I know people that have 12 foot plus probes. I have a 5 foot and it works most of the time. Depends on what you are doing. You could just get a length of round spring steel and weld it to a 5 inch piece of 3/4” pipe. Depends on how crafty you or a friend is.