I was originally thinking of titling this post as 'Using film with digital cameras' but thought that was a bit off compared to what the post will actually be about. What I want to talk about is using filters inside your camera, placed over the image sensor.
In a way, this is quite like using film. The filter is not easily removed / changed, so you are stuck with the filter for a whole shoot. Just as once you'd loaded a film, then you'd stick with that roll until it was finished. The types of filter you need to use are also thin and plastic, like film.
For this to work you need a camera that has an area around the sensor that you can attach your filter to. That puts all DSLRs (probably) out of the picture. I'm not sure what other mirrorless cameras are like, but with my Fuji cameras there is an amphitheatre like area around the sensor. It doesn't leave much room for attaching a filter to, but just enough room to make it do-able.
For the filter you want one that is thin and can be cut to shape to fit in the small area over the sensor. Wratten gelatin filters would probably work okay, but Lee poly filters are a better choice in my opinion.
To cut the filter to size, I started off using a circle cutter. I cut a circle of paper, then put it in the camera to check if the size was too small or large. When I'd got the size reasonably close (erring on the size of too large), it was time to cut the filter.
I placed the filter on a piece of paper, and one of the circles on top of the filter. The paper circle on top is to stop the central foot of the circle cutter touching the actual filter. Then I used the circle cutter to cut the filter.
With this done, it was a case of trying to fit the filter, trimming it down, trying again, trimming, etc. until it fitted. For attaching the filter securely in the camera I just put some double sided tape on the flat areas above and below the sensor.
You want to make sure your filter can be attached securely rather than just putting it in loosely. If the filter came loose while you were using the camera, one of the edges could get caught in the shutter, which wouldn't be good.
The main reason for adding a filter in this way is if you have a full spectrum converted camera. You can easily change your camera to a pure IR or red + IR or red + green + IR camera just by installing a filter like this.
Lee polyester filters tend to be quite a bit cheaper than glass filters. And having the filter mounted over the sensor means you can change lenses easily without worrying about also swapping the filter over.