Every Nagler-type 82 degree eyepiece I've used including Naglers, Meade UWAs, Luminos and ES 82 clones have the tricky eye placement issue. Some are better, some are worse. Interesting how the 100 degree "tall" eyepieces like Ethos are less finicky in this regard.
As for Nagler 31mm vs the clones, the Nagler uses "four exotic glasses." Do the clones? Probably not.
I recently rebought a 31mm Luminos. I liked the views the first time but decided it was too big and got a 26mmT5 instead. Then Usquebae mentioned the decloaking and one came up cheap ... The decloaking is easy and wow as you can see in the photo with the slim decloaked Luminos in the middle and shroud on it's left.
I am using an Agena 40mm eyecup http://goo.gl/BZhgTq upside down for now. A standard 2" cover fits well on the top (not shown). If they get the 48mm back in stock that would probably be ideal.