I'm sure we could come up with a few ways of generating entropy given what is available in a prison. Bear in mind this is for D&D so need not be completely formally random.
You can easily create a reasonable n-gon (for low n) on card or paper and add an axle from wood or whatever is to hand and you have a Dn spinner. D3, 4, 5, 6, 8, and perhaps 10 and 12 will work too - yes more than the usual but why not?
If you have paper and pen and something that will give you heads/tails ie 0/1 then use binary and generate tables of numbers and use them one by one. Other bases and simple devices can be used to get tables for a desired "die" max value.
I think the point is that we're not really good at being random, but combining two uncorrelated "random" draws into one gets something that will seem pretty random without closer inspection.
The point is that if a player needs a 6, they will always pick a 6. It's hard enough for people to come up with something random, but even harder if they know what number they need. Put two people opposed picking a number like this and it becomes random ish.
Well, dnd is largely cooperative: if you want to get a 6, I probably also want you to have a six. And if we're not even assuming that we are motivated to produce random results, than I seriously question whether such a simple rule would fix matters.
You can easily create a reasonable n-gon (for low n) on card or paper and add an axle from wood or whatever is to hand and you have a Dn spinner. D3, 4, 5, 6, 8, and perhaps 10 and 12 will work too - yes more than the usual but why not?
If you have paper and pen and something that will give you heads/tails ie 0/1 then use binary and generate tables of numbers and use them one by one. Other bases and simple devices can be used to get tables for a desired "die" max value.