Years ago, National Semiconductor sold a DIL - packaged "Noisy Diode" device that essentially consisted of a noisy diode and a comparator set to trigger within the noise threshold of the diode, resulting in an output that was not dependent on any other system parameter, such as time since power-on, or other effects. This could be run through a serial-to-parallel process, after some time-domain filtering, and assembled into random bit sequences of arbitrary length. It didn't depend on random timing of a keypress, hence, could be used unattended.

Maybe this will help. Nearly any sufficiently noisy device could be used, BUT, you must guard against the influence of power supply fluctuations within your system. This doesn't mean you have to spend a lot of time and effort on filtering the supply, though that's always a good thing to do, but, rather, you need to ensure that the diode noise is the primary influence on the output bit stream.

RE