Here's an idea. If you want to keep your siblings out, why not just lock up your box somewhere when you're not around? Or, if they touch it, just beat the crap out of them. Usually works. As far as installing a keyswitch, it might be near impossible to mount it in the front. I do like the idea of having it in the back to prevent power coming in from the rear, as a type of tamper resistant on/off switch. If you want to put it in the front, try looking for a momentary single-pole single-throw switch, desoldering the old power button, and soldering in the new one to the sitchboard in the front. Not an easy task, but it could be done.