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I've had the same problem, so I decided to make my own diopters for nearly all of my cameras.
All that you need is a frame that fits the camera. This is either the round screw-in kind or the slip-on kind. Next, you need a cheap pair of reading glasses that are the desired diopter.
You can get these at most drug stores. The small lens variety are preferable. And, you want plastic lenses so that they are easy to cut.
Remove the lenses from the frame. Cover them with masking tape on both sides, to prevent scratches in the process.
Mark the outline of the frame on the tape with a pen or pencil in the center of the lens. Cut out the shape with whatever tools you have. I use a jig saw. A coping saw will also work. stay away from the line.
Finish the shape with a small file so that the lens just fits the frame. If it holds itself in place, fine, your done. If not, use a little clear nail polish around the edges. Apply carefully with a toothpick. Now remove the tape and clean the lens.