A quick caveat regarding the possibility of using Rubylith as a Duplex filter replacement. The Rosco filter sheets are made of a rugged heat-resistant polyester material as they are intended to filter theatrical lighting. Using them in a Duplex should be right up their design alley.
But Rubylith I'm not so sure about. I don't know what it's made of. Or the coloring agent that is used. Or how those may stand up to the prolonged heat from close proximity to an LPS lamp. I've never tried using Rubylith for a six or eight hour darkroom session, let alone a dozen of those in a row over time.
Remember also, the Rubylith cuts out a lot of light because it's absorbing that energy. It is not a dichroic medium. And absorbing energy means it will get hot. So at a minimum it may fade somewhat quickly. Or at the other end of the scale it may fail catastrophically. Or it may be just fine. I don't know for sure.
Just something to keep in mind...
When I receive the other filter samples that Greg is kindly offering I intend to continue testing to try and get to the bottom of this. Unfortunately I don't often have large blocks of free time, so it may take awhile. I can post frequently to APUG because I sit in front of computers writing software all day. But I don't always have big chunks of disposable time otherwise.