I do the small Dixie cups like you see at water fountains. Fill with water and freeze. Push them out of the cup and into the water. Not much ice required. As many frozen cups as you want. Fill the cups up and do it all over again.

My knees were done 12 years ago & didn't have an ice machine. I just recently had my shoulders replaced. The first one I had to go hunting for an ice machine not even 24 hours post op. It was so worth it. I used a similar thing but instead of pop bottles I used blue gel bricks like you would use in a cooler. I had enough to have one batch in the machine plus 2 sets in the freezer as they take longer to freeze. Those plus water was wonderful in helping with the
pain. 7 months later when I had my other shoulder done (at a different hospital) they sent me home with a machine. The brand i used is DonJoy. Just make sure to measure the ice chamber so you can fit the right number of ice blocks in it. Mine took 2 medium bricks. Put in the freezer ahead of time so it was ready to go when I got home!!!