Chris,
As a physical therapist who also has a long history of lower back pain, I can certainly sympathize with you. Standing for long periods of time can really be a bummer for people with low back pain. I wear a Simms Backsaver belt and have found it to be helpful. Cabelas also has a back support wading belt that is about $10.00 less expensive than the Simms and that looks like it should work. Although I have not tried one, the Simms Back Magic belt looks like probably the best of the lot. It is more adjustable than the others, which allows for a custom fit. It appears to be copied from one of the better back support belts I have seen in my journals.

Larry Compton