Peter, perhaps have a look at the Wratten 2E filter - it might possibly do what you want, but I have no idea where to get one. The standard filters are gelatin, but someone may make a glass one - quick search did not find any though, so will keep looking.
Agree with Eric - is probably worth persevering a bit with filters if there is still some possibility of getting a usable system - the idea of using the two scopes is a really good one.
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