Petroff filter frame size question

[Eric F Adams]

Alright, I need you experts to give me advice regarding my Petroff MB-6 Matte Box. During my last post, y'all talked me into to getting filters for when shooting in daylight on 16mm. My matte box never had filter frame holders, so in the past I had taped over the openings. I recently purchased a 4x4 filter frame from a company that assured me the frame would fit my matte box. I had sent them photos. Well, when i got the frame, it was a hair too big. You can see in the photo that the 4x4 didn't fit. I contacted Petroff to ask them what size goes with the MB-6 and they wrote back that they quit making that 25 years ago. I replied to them I was aware it was old, but if someone wanted to acquire a filter frame for it, what would they get? They did not reply. Would anybody have any suggestions?