I'll second what John copy/paste above.
I will also add that even though you can build one our of harden steel, I can tell you from experience that it won't more then one knife's worth of cutting the shoulders on a hidden tang. I know because I did it.... with 5160, 01, 1095, and even tried some Cruforge V once. All of them were wrecked before I got the shoulders completed on a single hidden tang blade.
Now I actually have several different carbide file guides for various purposes in the shop. My favorite is one of Bruce Bump's, and others that have the carbide dovetailed in place. I also have a couple that have the carbide "glued" on, and have found that they tend to get "off" depending on temp.... which I suspect is the glue expanding/contracting with temps.
I have different ones for different uses. In my experience, the wider the carbide "slips", and the closer they set to a blade clamped in the fixture, the easier is is to get a flat, true cut with a file. As the carbide gets more narrow, and further from the blade clamped in the fixture, the easier it is to get a convex cut due to file flex, that most don't realize they are doing....until they try to fit a guard and see hairline gaps all about a given should.
I spotted it somewhere else in this thread.....and totally agree with it. For the cost of a modest knife, you can buy a good carbide file guide. It's more then worth it!