3D Printing

I used tungsten carbide end mill bits, 4 fluted, at low depth of cut and low feed rate but about 800 rpm, on a mill

For fiberglass cutting, you'd typically use a carbide, diamond toothed bit / burr. . Note that this doesn't refer to the cutting surface being coated with artificial diamonds (different purpose), but rather the tooth shape further up the tool shank. These burr-type tools exist with different cutting tips as well, I typically use a regular endmill type.

Here is an overview: https://carbideprocessors.com/ata-tools/fiberglass-cutting/

Note that up- and downcut bits exist, if you're cutting thin plate stock, you often want to use a downcut bit to prevent the cutter from trying to tear the stock upwards during cutting and ripping bits and pieces out, or tearing the stock out of its fixture.

I assume this is will be a one time job, so just get a cheap import bit - it will only be good for a handful of cuts, but will also only cost a few bucks. If you want some mid-range, decent cutters, you can look at

https://bitsbits.com/product-category/whiteside-router-bits/straight-cat/fiberglass-router-bits/

If you're doing this freehand with e. g. a router, do yourself a favor and go outside. In any case, even outside, wear a proper mask (at least N95 rated) - you'll produce fine epoxy dust, and breathing that in is linked to all kinds of lung diseases, plus cancer. No joke, overexposure to that stuff really turns your life into a miserable shit show.

If you have a CNC, I assume you know what you're doing, have proper dust collection etc. Even if so, still wear a proper mask if it's not an enclosed machine with certified dust collection.