I used this box for casting tin.

A model placed within, then filled up with molding sand - this works well. Beware that the plastic melts, where it touches the hot metall. But don´t bother about it - it will work anyway. A more detailled description (german) and more pictures can be found here: http://www.3d-druck-community.de/thread-4373.html

I place a freecad file as well - so you can change the size.

I released a casting ring as a customizer as well: http://www.thingiverse.com/thing:1882985