If you run out of ice - just fill up a very clean white plastic bag with water - tie tightly - you may have to double up on the bags - put in freezer and out comes a big ball of ice. Smash with hammer - Enjoy
The trouble with putting water in a bag and tying it up is that you will get creases and then it is nearly impossible to get the ice out without lumps of plastic. I'd suggest using tupperware tubs and only putting about an inch of water in the bottom. You can then sit it in a bigger tub of warm water to release the block. Then you can start smashing it with a hammer.