Back when I used to use a file jig, once I got to the second bevel on the opposite side I would use something to solidly shim under the tip and belly area of the underside of the blade to keep it from flexing and to keep a nice solid surface to file against to remove any chattering. You can use a scrap of wood or the like that's about half the thickness of the steel, just enough to keep the blade solid but not so much that it would end up raising the tip up and end up changing the bevel angle.
Also, I honestly don't think there's anything wrong with a file jig. They have some quirks to work around but plenty of people start out using them now days (ever since Aaron Gough popularized them with his YT video on how to make and use one) and a lot of them get decent results. I do agree that's it's good to also learn to work freehand for when a specific blade will benefit from freehand filing, but for smaller knives and crisp bevel lines they work fine.
I made these illustrations a number of years ago for somebody (I forget who) that was asking about my jig and how it was made, which was before Aaron made his video so it's a bit different than his design, but I think adds a bit more flexibility with room for moving the file around. So if you're interested, here are a couple pictures of how it's put together and how it's set up etc.
It's just two lengths of square steel tube with two lengths threaded drill rod going through each end of the tube stock, and a couple of bolts holding the bottom one to the bench and beneath the top bar to keep it at the desired height for the file angle. Just ignore the wooden stick I used in illustration to attach the file to and instead use a firm steel rod or tube of some sort. The rod/tube being round help allow you to rotate the file just a bit and the belly and tip area of the bevel.
These are the only pictures I made which only show the side view. I have an old YT vid back from 2012 with some footage of me using it so you can see how I have mine set up. Here's a link to the vid in which you can see the jig starting at around 1:19 into it: (Just add the www. to it, I don't want to bog down the page with some old video of mine)
youtube.com/watch?v=mUyoaXMQDy8&t=212s
You can also see what I used as a guide to file in my plunges, which was just a small piece of bar stock I had between the C-clamp and the blade.
It's not pretty but it got the job done. The drill press vise I have the 2x4 clamped in that the blade is clamped on was my main vise I used for cutting up steel and bar stock with my angle grinder, and it's all attached to a wooden saw horse I could take outside to keep the grinding dust and sparks outside. I didn't have much room in my shed at the time so I had to get as much use out of that saw horse as I could, so I used it as a work bench at times as well as for my file jig.... hence, it's ugly lol. Anyway, it should be enough for you to get the concept, which I'm certain can be improved upon. This may be completely useless to ya but it just might help, so I figured I'd share it with ya.
~Paul
My Youtube Channel
... (It's been a few years since my last upload)
