AP Barcon and 48mm Filters

OK, maybe I'm too stupid to own the AP Barcon 2" Barlow, but I can't figure out how to use a single 48mm filter cell with the Barcon lens cell and Baader binoviewer. Whether using the prism diagonal (refractor) or plain nosepiece (Newt), there is no way to thread a 48mm cell into the light path.

What am I missing here? Duct tape?

I believe the only way to introduce a standard 48mm filter into a Barcon/Binocular Viewer set-up is either:
-via threaded extension tube ("ahead" of either Barcon lens assembly or complete Barcon)
-via threaded tapered nosepiece (with or without Baader diagonal - "behind" complete Barcon)

Other custom options (adapters or coupling rings) are possible, given thread compatibilities (Barcon, filter, custom adapter, diagonal housing &/or bayonet ring).

Hope that helps.
Best wishes and luck.
-Dan