Which red lens are you using? I've been thinking about doing this for months, but still haven't. I've used 13060 since September on my nightly rides and it's never let me down, so if it's decent with a lens, I might just buy another.
I grabbed the first piece of red plastic i could find for the quick test - a lens off an old cateye tail light, which was wide enough to cover the 40mm diam lens.
Obviously a nice round 41 mm lens would be better, ideally one with threads to fit the flashlight. I wonder if there are any useful inexpensive filters out there in photography world. Hey I think I got a couple that are ~55mm, for the old FILM slr. Maybe I'll try those.
I don't think you'll get a dinotte-like red light regardless of filter you use - the blue component of the P7 lights seems to turn red-filtered light into an orange-ish color. Which is probably OK anyway. And still less then 1/3 cost of a dinotte tail light, with additional benefit that it can be used as a flashlight.
Please let me know if you find anything.