I've been tasked at doing some R&D for a fundraiser this August.
The idea has been done before and I know many of you would have seen this growing online for the past few years, and it's apparently still hip. So here's some documentation of how this will turn out.
Ratio of cement to water is 2 : 1.
Mix it till it feels like when you scoop peanut butter with a knife.
Ensure that your containers are coated with vaseline/oil (I used vegetable oil here) before pouring your cement mix in.
Light weight option, mix in some styrofoam balls or perlite.
To those who are interested, this is not the first time I've worked with cement.
I had a huge pile ordered before, along with sand for this work.
So I felt it was quite nice to work with it again (with budget too!), and it does feel quite therapeutic.
Try it yourself and have fun when you do.
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Updates 17/04/2016 :
Perhaps 2 containers for the mould is sufficient. Once you add another into the second container, it sort of adds to the contraction of matter and thus results in all containers being very difficult to remove.
Also, Sardine tin was very difficult to remove, because of the extra edge at the bottom, so was the milo ice cream container, for the same reasons.