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(wouldn't recommend more than 9v)
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I run mine on an external 7.5V linear regulated supply to keep the voltage drop across the onboard regulator as low as possible. Still gets quite warm, especially when the original SOC VDU and its RAM expansion are also being powered through the onboard regulator.
However the Bipolar PROMs run far hotter than the heatsink does..
Edit: No Colin, there never was an official case for the MK14, people extended the display, keypad and reset switch off the PCB and built them into all sorts of enclosures. For a time mine lived in a smart executive briefcase with the PCB etc in the lower half and an opaque white perspex sheet over the top with a calculator body mounted on it. The calculator keys were wired up to the appropriate connections on the keypad edge connector and the display was positioned to 'shine' through the red window on the calculator body. That's the smartest my original MK14 ever looked, actually, it's a shame I don't have a photo of it like that.
If you do go to make an actual custom enclosure then it will need a lot of ventilation around not only the regulator but the PROMs, which run bl**dy hot, and also it will need cutouts to allow edge connectors to be plugged into the rear and side edge connectors.
Try to imagine what Rick Dickinson (designer of the ZX81 and Spectrum and Spectrum Next) would have made it look like if he had been given the job. Let's see some concept art sketches...