ITPython, don't give up so easily. It's not as hard as you make it out to be. I've cut out quite a few of these, and all it takes is a bit of patience and some time.
First, you will need a sharp pair of scissors. I'm too clumsy to use as knife, as I'd just end up cutting myself.
After you've picked out the filter, simply place the flashlight bezel down on the filter and using a pen, draw a circle around the outside of the bezel onto to the filter, then cut out a small square that contains that circle.
What you do next will depend on whether you're going to mount the filter inside or outside of the front cover.
Of course inside is the best choice if possible, and if you do that, then it's easy to simply take off the front cover and lay it on the filter and draw another circle around the cover. Then, taking your time, start slowly trimming away that excess filter until you just have the part with the pen drawn circle.
The way you trim, is to hold the filter in one hand and make a straight cut with your scissors all the way across the filter just to the edge where the line is, then you rotate the filter a bit and make another straight cut just to the edge. Each time you make a cut, it only gets just down to the edge where the line is, so you need to do this rotate and cut thing about 20 times to get a rough circle, rotating it just a tiny bit each time.
Think of it like this. Draw a circle on a piece of paper, then using a ruler, draw a straight line on the paper that touches the outer edge of the circle. Then turn the ruler a bit and draw another straight line that just touches the edge of the circle. Keep doing that until you've touched all the edges of the circle with a straight line.
That's what you are doing with these cuts. You are slowly cutting out the circle using straight cuts that each time only barely touch the edge, and after enough of those you will have a rough circle. Then it's just a matter of
SLOWLY smoothing it out and adjusting the size.
It's not hard, because your cuts are straight, and after the first time around it looks really rough, with all these little points sticking out, and you'll still be able to see the circular line you've drawn, so it's probably still a bit to big.
Now, place the filter on the glass cover and squeeze at the edges. If the filter is to big, it will buckle a bit. So, keep
SLOWLY trimming, making straight cuts and rotating, but smaller and more precise now, making the circle smoother as you go, and stop every 2 or 3 cuts and put it on the glass again and see if it buckles when you squeeze the edges.
Finally, you'll get it where it fits on the glass just right, then clean both the filter and the glass, but remember, this is a coated glass and needs to be cleaned VERY carefully to not mar the anti-reflective coating. I use the special tissues and cleaner for my glasses and it works well.
Then re-assemble and your done.
If you're mounting it on the outside, it's basically the same procedure, but you just have to take it a little slower, and check the fit more often, and at some point it will just fit down inside the bezel and not come back up. I've done 3 lights that way, and while it wasn't hard, it did take me about 30 minutes of patiently and carefully sitting there and slowly trimming and then checking, and then a tiny bit more trimming and then checking, but I never had one that didn't fit perfectly in the end.
So, the key is a pair of sharp scissors, and patience. I put on some nice jazz while I'm doing it and make sure I'm not in a hurry. Also, make sure your work area is clean, and I laid down a white T-shirt to make it easier to see when I dropped the tiny filter, which I did a few times.
It really is worth the effort, and if you have a few mistakes, then keep trying. Even though I've done this quite a few times now, it still takes about 30 minutes for me to slowly and carefully trim these.
I hope this helps
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