If you were to use a 6 sided dice, you could roll the dice 8-byte/6-dice * 128 = 171 times and convert a base-6 number to a 128-byte base-8 number.
You only need to roll a dice 99 times to get a 256-bit number. Which gives you a bitcoin private key.
If you wanted to be even more random - evenly distributed and independent - with dice, you could get a 20-sided dice but then collect the data in base-2 by saying every even number was 1 and odd number was zero,
The thing is, a 20 sided dice has more biases than a 6 sided dice. The more sides the more bias. So that might not be a good idea.
Physics is such a beast that there could be a discovery on how to make radioactive decay not-random.
you can't cause or influence when an atom undergoes radioactive decay. plus you don't know exactly when it will decay. look it up.