if people are looking for another product that's mould-able, good for repairs and doesn't go off (but also doesn't stick posters to walls) then I've done great things with Polymorph.

It's a thermoplastic that sets solid can be fused together and reused by heating it to 62 degrees centigrade. It's also non toxic which makes it great for kids.

http://www.preproom.org/equipment/eq.aspx?eqID=5105

I always used to get it from Maplin and can't find much online, but it looks like there's a whole range of thermoplastics now available.