I've never made one, but the first thing that comes to mind is that you could use clear gelatin in ice cube trays.
Hi there, I have used both knox and rock candy. My thoughts, Rock candy can be expensive and I did not think the look was right, I used the Knox and tried the ice cube trays and it did not set up so well, Then I used a square pan poured in the Knox about 1" deep and cut into cubes after it set.I found that this held up well,and after a test on the counter 18 hours it finally started to weep a little.was not worried about 4 or 5 hours on the cake. My ice cube cake is in my pics. Good luck.
This is a very good question, I made a beer basket (posted in my pics) and I resulted to using ice cubes that I found at walmart, although they were very heavy and I had to be very careful when I positioned them so that the buttercream would not get so smushed (if that is a word 
). I would think that the clear gelatin would melt on to the cake or at least flavor the icing a bit .
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