I use old cameras including a Leica IIIf and a couple of Zorkis. The problem with bottom loading cameras is that the film can be difficult to load. There are one or two suggestions on the net, and of course there is the official Leica tool the 'ABLON' which costs a couple of hundred £, $ or €. There was a cheap copy made but that is ridiculously rare. So here's mine, made in about an hour.
1 sheet of plastic card 1.25 - 2mm thick, modellers glue or epoxy resin, the template.
Printer and paper, craft knife, fine file, tweezers, fine saw, clamp or vice.
Step 1: Template
This is the template I shall be using. Download it and resize to 13cm wide to 12cm high. This shall be the exact dimensions needed. Print it out.
Step 2: Cutting the Plastic
I use plastic card about 1.25mm thick, any will do as long as it is stiff. Cut 2 pieces of plastic off the sheet, 35mm by 115mm. I'm hopeless at cutting exactly so I made mine a little bit longer and wider, clamped them together and sanded the edges until they matched. If the plastic is shiny then just sand over it quickly with some 600 or 800 sandpaper until the shine is off.
Cut out the film strips. Rub a pencil over the back of the print following the cutting guide. Do it also in a couple of places where the perforations are. The pencil acts as carbon paper. Line up the print and one of the plastic pieces and draw over the print and the perforations you have selected. This transfers the template to the plastic. Do the same with the other strip and plastic. You should have two pieces, a mirror image of each other.
Clamp or sticky-tape the two pieces then carefully cut along the template. It's not an exact size, but make the rounded corner nice and smooth. I put mine into a small vice and sawed it. Then scored the curve with a craft blade and bent it gently until it broke.. With the two pieces clamped together, file the cut out smooth, I used a half-round file to gently smooth off the curve. Sand the edges smooth.
Step 3: Perforations
Line up a film against one of the parts and watch the perforations line up against the marks on the plastic. If they do, that's fine, if they don't - well draw a couple more with the film as a template. Cut a thin strip of the plastic, 1mm wide, you don't need much. Cut off 4 or 5 1.5mm pieces. These will go through the perforations so they can't be too big. They are fiddly, so I used tweezers. Put a tiny drop of plastic modellers adhesive or epoxy glue and put the tiny pieces on to the glue, inside the perforation hole mark. If they are not in precisely the position, it is possible to file them to fit once they are stuck down. Sorry no photos of this operation as I would need 3 hands! I call these the pegs.
With a piece of film, check that the pegs are in the correct position, sand them gently until they fit.
Mark the opposite strip with a pencil showing exactly the position of the pegs as a hole needs to be made to accommodate them. I made the hole with a 1mm hex key heated up and simply melted the holes. File down the edges of the hole with a small, fine file and put the two pieces together. Adjust until the pegs fit snugly - but not too tight into the holes..
Step 4: Finishing and Using
I stuck my two halves with sticky tape in the middle. This will do the job.
To use, line up the film, right to the edge of the template in the position shown. make sure the pegs go through the film perforations. Close it together and the film is locked into place. Cut the film with a penknife, taking care not to damage the plastic. I must get a tiny one to use for the purpose.
Alternatively mark the film in the template then use nail scissors to trim it.
I usually cut a few films at a time so I'm not messing about when I'm shooting. The cut films work as normal in any other camera.
Be careful there are no sharp edges and that the perforations are not cut. They will break in the camera and are difficult to get out.
The exact thickness and shape of the film 'tongue' is not an issue, but there must be a smooth curve and no broken perforations.
I have seen the metal original and I think it could be fabricated out of a piano hinge
Here is a demonstration on how to load your Leica, showing the use of an official film cutter: for some reason I can't get it to link. copy and paste this: