The downside of Sugru is the time it takes to set.
I tried out 'Siligum' from Hobbycraft and other places as way of making silicone moulds for low-temperature moulds for white metal. It works really well and the final result is very like sugru, but it only takes five minutes to set, you also get more for your money! It's a two-part compound rather than being air/moisture set.
The downside is that it is not sticky so you need to mould it to get a mechanical join. I'm doing an experiment with some silicone based glue to see if that will adhere to it.
Neil