This sounded simples. Get mega blocks (large duplo), clean, fill with jelly and set. Hey presto - lego style jelly.
In reality, it didn't quite work. Mega blocks are actually divided inside, so even the largest pieces only created 1 small piece of jelly. And they weren't the easiest 'moulds' to release the jelly from, so despite dunking briefly in warm water, they took a while to make that lovely slurpy sound and slip out, and by the time they did, the tops of them had melted somewhat and that distinctive lego shape had disappeared and turned into a more rocket shaped lump.
We managed one or two pieces that looked as they should have, but we had to explain to Mini what they were supposed to be, as it clearly wasn't obvious!
In case you want to give it a go - just make up jelly as per the packet instructions and fill your moulds as you usually would. Perhaps a light oiling might help?