This Instructable shows how to create a 6-button silicone rubber keypad. If you are not knowledgeable about silicone rubber keypads, the Wikipedia article provides a pretty good description. I recommend you read it before proceeding.
This Instructable contains STL files you can print on a 3D printer, both for the mold to create the silicone rubber keypad and also for the top and bottom plates that hold the keypad in place. It is intended to be incorporated into designs that contain a processor such as an Arduino or a Raspberry Pi, but is not a standalone device.
Use a 3D printer to print the two attached STL files. The resulting objects are the two halves of the keypad mold. I printed them in ABS plastic, but I see no reason that PLA or other common 3D printed materials will not work just as well.
Once the two parts of the mold are printed, spray them with mold release. Mold release does for the molded object what greasing a cookie sheet does for cookies; it makes it easier to remove the object. A light coating is sufficient. I recommend you spray in a well-ventilated area.