Good point about the film base, Donald. Do keep in mind, however, that in my previous tests, I had film base + the thickness of the transparency film involved. The coor shifts, I also think, were partially a result of inproper alignment of the color screen mask. I'll try this & see how it goes.

Also, when you say sticking them together with the gelatine is a good idea, will this be permanent, even through processing? It needs to be completely watertight; water causes the dyes in the screen to bleed, developer does even more so, and dichromate + acid, I don't even want to think about ...