If there is a vending machine with ice or if it’s freezing outside there is no problem. But today I’ve managed to get some ice using the weak “fridge” in the room.
As I said here, neither hotel fridges are powerful enough not they are intended for freezing anything. Cooling is more than enough in most of the cases. However, there is a way.
Pour some water into a small thin plastic bag and tie it off. Now slowly put it on top of the ice in the fridge. There is always some space between the cooling element and the top. If it does not fit try to “pour” it on, but beware of any hidden cavities. If the water gets in and freezes the bag will stuck.
My test succeeded. It took about 5 hours to get ~100-150ml of ice. Usually enough to hang a knife or keys on and enjoy inescapable selfbondage for a couple of hours.