Hi, thanks for questions.
1) Filter sizes, it could be an options, though making something to fit firmly is tricky, measuring filters around various rentals and brands, they differ. Existing design will fit most of filters i have seen, for example clear crash filters are 5mm thick, NDs 4mm.. Older Tiffens may differ up to 2mm in dimensions.. I would just make specific MB for such filters.
2) 100x100x2 filters should fit, thanks to clamping design.
3) haven't thought about side flags, for most situations with side flags I tend to use LMB or studio styl mattebox. This is made pureley with thought to be light and easy to use with Intension to replace filter taping.
4) That type of design is in process, though need to sell some mattebox to buy 3d printer that can print 2 different materials and then combine rubber with hard plastic to create working design.
For still lenses you can have smallrig or tilta with thread adapters.. plus even 4x4 filter in most cases is to heavy for photo lens and mount itself (except of course for locking type.. Though could think of 15mm rod adapter version to use with still lenses.
5) !!! HUGE PRICE REDUCTION as well 4x4 95mm coming soon!!