Phil,
About ten years ago I decided to stop squinting and start watching with both eyes open. Rather than using an eye patch like a pirate, which has the drawback of dark-adapting the idle eye whereby going back to binoculars takes a while, I adopted the trick biathletes and some other shooters use. This is to attach an obstruction in front of the idle eye, close enough so that eye cannot focus on it but far enough that stray light prevents dark adaptation. The first one, for my Fieldscope back then, was simply made out of a bit of copper wire holding a black disk about couple of cm in front of the eye, fastened around the eyepiece base. Now, on my ATX, I have a similar thing held in place by the rubber ring that holds the eyepiece cover. They have a black surface towards the eye. Neither looked very elegant, but they work fine. Enough off-axis light enters the eye that going back to binoculars causes no difficulties whatsoever, and with the scope I can view for as long as I please with no other discomfort except some neck strain that comes from viewing down into an angled scope for more than 45 minutes.
I'm planning on making a more elegant version of this soon, and when I do, I can post some pictures. Highly recommended.
Kimmo