I added the wrench mod to my second BRS-3000T stove, and here is the process I used.
First, let's agree on some terms. In most of the pictures below, the pot support arm is shown in the same orientation. I'll refer to the three sides of the triangular cutout as left (short side), top, and bottom (hypotenuse).
Unscrew the burner section from the valve body and remove the jet. If you want a particular one of the supports to be modded, mark it before removing the burner. Go ahead, I won't judge you.
Getting an outline of the material that needs to be cut out was the trickiest part. Since the bottom side of the stock cutout is the thinnest, I kept it unaltered and used it as one side of the wrench. The photo below shows the location where the new cutout will be. The hex of the jet is flat against the bottom of the triangle and moved as far left as possible.

You might notice that it is difficult to hold this little jet in place. I used some aluminum foil tape to hold the jet for me so I could mark the cutout profile. Foil tape is good for this because it stays in whatever position you push it into. I put a small piece of tape on the jet as shown below, leaving about half of one hex side exposed for aligning with the bottom edge of the triangle.

Now tape the jet onto the support, making sure to keep the hex flat against the bottom edge and as far to the left as possible. Here is what it should look like from the back side.

Now, back on the front side, use three pieces of good masking tape with straight edges to very closely mark the top three edges of the jet's hex. When you think you've got it, look at the backside again to make sure the jet is still properly aligned.

Now with the jet out of the way, you can see the outline of the material that needs to be removed.

Rub the edges of the tape down well, being careful not to shift their positions. Recheck the outline again with the jet and then fill in the outline with a permanent marker.

Remove the tape and behold. If you rubbed the tape down well enough you'll get a nice sharp outline like this. If not, well, practice makes perfect (the ink will come off with rubbing alcohol).

Recheck your outline with the jet one more time.

Time to start removing material. I used a set of needle files like the ones shown below. This is a cheap set of 6" long files from Harbor Freight or somewhere.

I started with a square file, the width of which was about equal to the length of one of the hex sides. Don't use a file with a side wider than that.

Work on the longest edge of the marked area first. Keep the edge of the file parallel to that edge as you go and keep the file cut centered with the longest edge.

Just getting started:

Keep going until you have almost removed all of the ink from the long edge, but not quite all, just in case your outline is oversize.

Now work on the short side to the right. Pick a file that has a non-cutting edge like this:

Keep the smooth side of the file toward the work you have already completed and knock out the right side, always staying parallel to your outline.

Again, stop just short of your outline.

Similarly, clean up the left corner. You don't have to take all of the material out of that corner as I did, but it looks better if you do.

Now it's a matter of checking the fit with the jet and filing out small amounts at a time until you have removed just enough material. When you get to the "almost but not quite" point where it feels like there is just a bit of material preventing the jet from going in, be sure you know where the hangup is before removing any more material. It would be easy to remove too much material from one side when all you need is to take a small amount off of another side. Keep the corners sharp and the sides straight.
When you get just enough material cut out it should look something like this:

You've created a lot of debris during this process so clean out the jet before screwing it back onto the valve body.

There is an easier but less precise way of marking the outline for the cutout, and it would require removing more material but if you find the method above too fiddly, you might like this better. Simply insert the threaded part of the jet into the support window as shown, keeping the bottom edge of the hex and support parallel.

Hold the jet in place and mark around the hex as closely as possible with a fine point Sharpie.


This outline is clearly too large because the marker tip can't get close to the hex but when you take the jet out and align the hex with the bottom edge of the stock cutout, much of the outline actually lines up pretty well.

In this case you would stay inside of the inked outline when you file and you would need to check your progress often to make sure you were staying aligned and not taking out too much material, but I think this method could work satisfactorily.
Happy filing!
-Stephen