This is what you are going to need.

A piece of higher grit sandpaper.
A 3mm Drill bit that you are going to sacrifice.
2 x bolts and nuts (not shown).
A drill to match the two bolts (I used s 5mm here).
A strip of aluminium from a beverage can.
Two pieces of metal.
Marking pen or scribe (Something to mark the aluminium).
Scissors to cut the aluminium. (These are too big as I mention later).
Hand drill (although I have a milling machine, I wanted to make this out of basic tools).

Optional / A nice to have: A Clamp (I didn't use one because again I wanted to use basic tools)

The first easy steps.

Start by drilling the three holes.
The two outer ones to match the nuts and bolts that you have. They must be wide enough apart to clear a blade so anything more than about 30mm will work.
The 3mm hole somewhere close to the middle - it does not have to be perfectly in the middle. If you have a clamp use it here to clamp the two pieces of metal together before you drill them. I didn't and my two pieces don't line up exactly.
I then sandpapered the top end of the 3mm drill bit so that it was flat and snapped it off. Effectively it's going to work like a scissors.
While I was doing sandpapering, I also cleaned to the two pieces of metal.
If you have access to a Dremel (or similar) rather cut off the excess of the 3mm drill bit. The fact that I snapped it created a problem which I come to later.
You want to end up with something like the two pieces in the bottom half of the image. The countersinking is optional. I counter sunk the two outer holes because that matched the two bolts that I was using.
Up to now the only mistake that you can make is that the two outer holes are not far enough away from the middle hole that they interfere with the blade edges.

The next steps - this is where you need to be more careful.

Cut a suitable size strip from the aluminium can.
A - Mark and punch the two outer holes. I used a paper hole punch to do punch these holes. Then assemble the jig, sandwiching the piece of aluminium strip.
B - Punch the middle hole using the flattened (snapped) drill bit.
It was at this point I found out that my drill was slightly bent from breaking it. It worked fine but I had to pull it out with a pair of pliers - so this is why I suggest that you cut it with a Dremel if you can.
I wasn't going to snap another drill bit so I used the pin that I already had and punched the rest of the small holes that you see in the image.
C- If you have an old Gibbs blade use that, otherwise you are going to have to do some careful measuring. I took a blade that I already punched and marked it out with that
D - The Marking pen works fine but I found that scribing it was better.
E - Cut where you need to. Also cut off the excess so that there is no aluminium sticking out. You want to end up with the aluminium similar to this.
This is going to determine both the depth and the centering of the notch. It was at this point that I found that the scissors were way too big, so I ended up using the ones on my Leatherman. The aluminium strip is soft and cuts easily.
The great part is that if you make a mistake, just cut another strip from the can and start again.
Now it�s time to re-assemble everything. When you do, put the pin so the aluminium lines up and then tighten everything up

How to use it.

Slide the blade into the jig.
Insert the pin and give it a smart tap. Repeat for the other side. I used the handle of the screwdriver in the image to tap it.
The top of the image shows one side done and the blade inserted before doing the other side.
Does it work - absolutely perfectly, all the time every time.
See the bottom part of the image. The blade is perfectly lined up in my Gibbs.