I've done it for fun with a few metal films, just trim a tiny bit, like selecting some 999Ω from the 1k batch to trim down to 1.000k few years after I read exactly the same, but only 3 zeros, I've just received the 121GW so I could try something with 4 zeros.
I used some small files and went slowly, turning the thing as I went so I extracted as little material from a single place as possible and then coated them with some epoxi. I guess higher values would be harder as the strip is much thinner so there's a better chance of extracting too much material easily.
The hard part is that you are heating and getting conductive dirt onto the conductive layer as you file, so you need to clean, wait temp to stabilize and then read, that make the thing tedious and slooow. For the final trim you could go slow, blow some ambient temp air and read faster, once they settle make a good clean, put some epoxy and they might be good, age them for a bit before trusting them.
Or just buy better resistors to start with. Some pretty usual way is to build a network with a trimmer in it, but the trimming range being pretty small, so the tempco of the trimmer doesn't affect much the tempco of the combined resistor.
Carbon might be easier to carve but as they come in really horrible tolerances you could very well select a value from a batch somewhere off the nominal. If you need 10X divider, 910 and 82 820 and 91 makes a pretty exact value, you just need to select the resistors from a batch and you will find some. X2 is easy, to equal ones, time 3, probably use 3 equal ones and do series of 3 (that makes self heating more even than parallel) or get some 1k and 2k. All standard values of E24 series and there might be a perfect combination for each ratio you want, just grab the calc and do the math... or an excel sheet.
JS
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