I no longer have it, but I did have a Low Rider for a while and experienced the same problem.
My solution was set the factory tensioner to act as no more than a guide, (ie. with the chain run through it virtually straight) and to install a rear derailleur to provide tension. Obviously something like an Alfine or Rohloff chain tensioner would work as well, and be lighter, but given the nature of the beast* I doubt if weight is a concern!
* I use the term advisedly - ISTR that mine weighed >50lbs!