My 'simple' answer is to get a standard button battery, almost as large as the opening for the PX625. Then, simply point your camera at a scene with a known exposure value (at the proper set film speed) and make the tiny mental adjustment that will carry over to each subsequent meter reading. Dollar stores carry such button batteries. Get the largest that will fit into the hole. The PX625 is, I think, 1.35 volts and the button batteries are, supposedly, 1.5 volts. The difference in corrective exposure value will be small and easily remembered.
You will probably have to be a mite creative with padding it with either aluminum foil of metal discs (or a flat spring) in order to ensure proper electric contact, but this is reletively easy to do. - David Lyga