Make your own Soda - - Cheap!

Step 2 Metalwork - make the plates

First measure and cut two plates of your metal.
Mine are about 1.5 inches by 3 inches.
Notice the image below. You are planning to make the bottom plate with a slot in it to fit around the neck of the plastic bottle below the "shoulder", and the bolts will run up through both plates to secure the top plate against the opening in the top of the neck. The top plate will have the hole for the CO2 to be delivered into the container full of cold water.
Before you get started, put the container filled with water into the refrigerator.

Then cut your plates.
measure for the u-shaped cut out.
The neck of the plastic bottles is almost exactly one inch, so my plate is cut out to about 1 1/8 inch, which is a bit large, but still works.

So then cut the u-shape however you can. I used a cutting torch to do mine. You can use a hole-saw perhaps to start then a hack saw to finish, or perhaps you use a jig-saw or recip-saw. Whatever you have.

Next you need to drill the holes for the clamping bolts. Stack the plates, and drill both plates at once. This way your holes will be sure to line up even if they are not perfectly spaced.

Last, drill and tap the hole for your CO2 fittings. Mine are 1/4" Pipe.