24d10 would be miserable, even with dice. I takes approximately 4 coin flips to approximate a d10 (need to ignore values [10,15] and add one. This works out to

24d10 would be miserable, even with dice. I takes approximately 4 coin flips to approximate a d10 (need to ignore values [10,15] and add one. This works out to ~100 rolls. For the more common case of a d20, you could do 5 coin flips, but doing 6 instead gives you much more wiggle room. In that case, you only have to ignore values [60,63] so you only have a 1/16 chance of needing to redo the coin flips. It would be tedious but doable (particularly if you had some system where you could prepare rolls while waiting for your turn). d4 = 2 flips guaranteed, definitely not bad. d6 = 3 flips expected. d8 = 3 flips guaranteed. d10 = 4 flips. d12 = 4 flips. For something more typical but still nasty, a Fireball with 8d6 would take ~24 coin flips. Also, if I were a prison DM in this situation, I would probably take the time to homebrew the system so it only uses power of 2 rolls, i.e. d2, d4, d8, d16, and maybe d32. I would probably do d16 instead of d20 since d32 would be rolling 5 rolls quite often and a d16 is closer to a d20. Overall, the difficulty would not be the coin flips but doing the math to keep track of everything. Even if they restrict you from doing that, another option would be to do some sort of manual pseudo-rng. That option would probably be faster if you have access to a scientific calculator.