There's a reason why low B / single string dampening devices do not exist. There's no substitute to make up for poor technique. It would be akin to having stabilisers on an adult's bike because you don't want to ride it properly. That said, I never really explored the 5 string properly when I played one exclusively for many years, I played it the same as a 4 string with a few lower notes. However, I developed my floating thumb technique well enough to keep the low B muted. Perhaps it's time to reconsider whether you're a 4 string or a 5 string bassist. I'm definitely a 4 string bassist. If I need a few lower notes, I have a hipshot Xtender for drop D, or I use a pitch shift in my helix to instantly retune to BEAD. There's a couple of stompboxes (EHX Pitchfork and Digitech Drop) that can do the same thing for you.