I ran
cat /dev/urandom >> devurandom.bin
which produced 31 MB of data in a few seconds. To get numbers from the Arduino, I ran:
stty -F /dev/ttyUSB0 cs8 115200 ignbrk -brkint -icrnl -imaxbel -opost -onlcr -isig -icanon -iexten -echo -echoe -echok -echoctl -echoke noflsh -ixon -crtscts
cat /dev/ttyUSB0 >> TrueRandom.bin
which took several days to produce 93 MB of data (rate of about 3.2 kbit/s)