It can be done, but you have to keep in mind that the diffusion film is not your only obstacle. Blades that are not intended for thin-neck sabers have their diffusion film glued to the inside of the blade near the bottom. The combination of this glue, plus the diffusion film makes the blade almost impossible to seat into the shallow socket of a thin-neck saber. So, please take that into account before making modifications that might not get you the effect you desire.
If that is not a factor, and all you need to do is trim the diffusion film, you just need to measure it out so that what you remove will leave approximately half an inch or so above the bottom edge of the blade to allow for the tight socket of the emitter. The cut needs to be straight, and that film is tough so it has to be something very sharp. I was able to trim mine using very sharp, heavy duty shears. Just keep in mind that you can always remove more if necessary, but you can never add it back, so be conservative in your first cut, and trim more away if necessary, rather than taking a bigger cut, and coming up short when it is done. That would result in uneven lighting towards the end of the saber, since gravity will force it to the bottom, and expose the area not covered by film. Also, when dueling, you will hear and feel it moving around if it is too short.
Good luck my friend!