In general the films used must be dimensionally stable because of the requirement for precise registration. The films should be as thin as possible. Also it is advantageous to use a film that can be used with a safe light.
BASE FILM:
The best base film for building negatives should have a very long straight line density-log-exposure curve and a very high silver content. These are the characteristics of Kodak Commercial Film 4127. No other film achievs the image quality that 4127 film provides. The problem with 4127 film is that it is currently only easy to buy in 4x5 or 8x10 sizes, unless one purchases an entire run from Kodak. [This is a problem.]
There may be several substitutes for 4127 film. A possibility could be Kodak Duraclear material. It is processed by the Ra4 chemistry in a tray or mechanical processor. It is available in a variety of sizes including some long roles. This film has not yet been tested.
MASK FILM:
An excellent film for the masks is Kodak Kodalith Film 2556.
It may be helpful to have the base film slightly larger than the masking film, although this will cost extra. For example with 8x10 film, make the image size approximately 7x9 and trim the masks to 7½x9½. This is because the image is easier to view on the light table with the mask on top of the base. This is done when registering by taping together. Having the top piece of film smaller gives room to tape together. Having the mask larger than the base can work, but not as easily. Having the mask and base the same size makes it difficult to tape the films together for registration.