All experiments that I have done with a red filter on a flashlight have always turned it into a floodlight (it scatters the beam). But what you want is a proper beam with a good throw.
Therefore, you are much better off using a flashlight with red LEDs. Don't waste your time and money fiddling with filters. Three years ago I bought a red flashlight for just £11.42, including a rechargeable li ion battery and charger, and it has an adjustable beam with a zoomlens, so it can make a nice narrow beam with massive throw. You'll never achieve that with a filter placed in front.
However, as I wrote before, I reverted back to a (much weaker) white light because I find it difficult to distinguish features in red light.
Nowadays I just use a (weak, white) headlight (the ones you use for hiking) and because it is handsfree I can observe the birds through my binoculars. This way I don't need much light and still see nocturnal birds reasonably well. I can use a low light level that doesn't seem to disturb the birds much. I have observed nightjars and potoo this way and they just went on with their business, catching moths and kept returning to the same perch. I kept the light level so low that I lost most colour vision, but I still preferred it over red light. Also, with white light you still have the option to switch to a higher level for a few seconds to see the bird's colours, which might disturb the birds, but at least you have this option and you can decide whether this is appropriate or not.
But anyway, red light may work for you, so just get a cheap red LED flashlight online and see if you like it. Even a cheap one will outperform anything that you can achieve with a white flashlight + home-made filter.