My Nimbus-3 was identical to Dale Bushes Nimbus-3 and we both had only internal seals. On an annual inspection, I noticed my internal seals were deteriorating and I was able to pull them out through the gap. I then installed external Mylar seals only. My ship remained identical in both climb and cruise to Dale Bushes. From then on, I stopped installing internal seals. My conclusion is that properly installed Mylar seals top and bottom are all that is needed to stop cross-flow and it is one hell of a lot easier! Not putting internal seals on the ship I'm finishing up right now.
JJ