I was thinking that they would use the squeegee method since we don't have the budget for custom acrylic. The grips are yet to be hired by production and I'm booked on gigs the next two weeks, so I may have to ask a friend to do the final inspection.
The window in question has a round top edge and it's a very old wood frame window, so it would be difficult to get something that perfectly fits. In my house I used the Gila spray product from Home Depot as the "adhesive" to tint some windows with Gila gels. It's not perfect, but it's suitable. Sticky soda would be a bad choice in Southern California. We have problems with ant invasions and this is someone's home.
The DP and director said they will be either framing wide in the room looking toward the window (establishing) or there will be action in the middle of the room looking at two characters having an argument with the window in the far background. So I think a few bubbles and imperfections will be fine. I think a scrim would be obvious in the wide shot. Besides, I think we're going to need at least 3 stops. The camera is an F3, so it's not like the Alexa in terms of highlight retention. We also have no big lights to blast through other windows besides shiny boards.
I just wanted to check if there is a more professional way to do this given that I've never ND'ed a window for a film project before. From the responses so far it sounds like we're on the right track.