Another method that involves just a little bit more work is to insert some battens in sliding dovetails.
Now there is a plane that is designed for that specific purpose, but mine is at home, so I had to do it with my smoothing plane instead which means that my battens visually taper and don't cover the line up as they would have if I made them the other way.
Since this isn't a show surface it will be just fine.
A narrow board was divided to form two wedges. The surfaces were cleaned up with the plane. Each of the edges were planed at an angle, so the end of each wedge resembled had a trapezoidal shape.
I marked out where I wanted the pieces to go and clamped down the first wedge. Using itself as a guide, I sawed along its edge using my small dozuki. When I had reached my intended depth I loosened the clamp and shifted the wedge a bit to saw the other side of the dovetail dado.
Once the sawing was completed I removed the material with a chisel.
A router plane would have been the obvious choice, but the body of my small homemade one is so narrow that it would fall into the dado. And a chisel does the job fast and well enough in this case.
The wedge was marked out so I could saw off the lower part of the wedge, to enable the protruding end to grip behind the lower front lip. Finally the edges were chamfered with a chisel and the wedge installed.
The second wedge was negotiated in the same way.
A board was split and resawed and planed for making the locking pin. It was cut to length and a hole drilled in the upper part to give something for the fingers to grip when it has to be pulled out.
The bridge shaped piece that will hold the upper part of the fall front was a quick saw and chisel job.
Ralph asked about the shaving deflector for the Stanley No 50 combination plane (mine is actually a Record plane).
As you all know, taking pictures isn't my strongest side, but hopefully the pictures of the deflector mounted in the plane will give a bit of an idea on how it works.
The backside is sloped to match the blade.
The inside is sloped to that the lowest point of the deflector is positioned as far outwards as possible. This slope guides the shaving to the centre of the plane where it can escape without being jammed.
The completed fall front.
Sawing the side of the sliding dovetail dado.
Resawing the locking pin.
Shaving deflector seen from the side of the plane.
Shaving deflector seen from the front.
Shaving deflector seen from the top of the plane.