Hi Jon,
I have a "tip" for your filter connection area.

Make a "middleman" connection piece as a seperate piece. This way, you can utilizes all the different filters alreafy made also.

So your mask has some type of connection, with a seperate piece, that may be made with any number of secondary connection types and sizes, per whats available.

So my 3m niosh mask has a 1/4 turn connection mechanism (kindof common, but size?)
You could just print the little adapter connection to make ypur mask compatable...

Hope that makes sense.