Why the Raspberry Pi Is Actually Pretty OK For This

• The Raspberry Pi is small. It can be locked in a safe when not in use, or even locked inside of a small faraday cage while in use.
• The Raspberry Pi is cheap. Even if you're totally paranoid, and want to destroy both the SD card and the Pi once you're done generating pads, you wouldn't be out an unreasonable amount of money.
• The Raspberry Pi has a hardware random number generator on it. This. Is. BIG. This means that, with no additional hardware, you can put the Pi to work generating truly random numbers, and doing it fast. Most computers only have software pseudo-random number generators, and therefore cannot be used to generate one time pads.
• Thermal paper printers do not secretly encode the timestamp and serial number of the printer you used onto every print like laser printers do.