Happy New Year to all. Again I have problems with 4-letter words!
24 Across: Some mainsheets... ins? Can't be insh surely?
28 Across: ...badly cut S?I?
Brain not quite prepared for 2017...
2d Film like movie not film.
Gone with Mask. As pointed out it is a film, but also in filming a movie, a matte or mask is used to "not film" parts of scene.
Lough Mask fits in nicely with theme too.