Got to disagree with the MDF rule, it's horrible stuff, structurally a nightmare and if bonded with certain chemicals creates a horrible dust [http://www.childrensfurniture.co.uk/mdf.html]. By all means use it if you want to but a 'just wood' rule sounds much better than just MDF to me, and would allow the use of a wider range of wood scraps/cuttings from around the house/skips etc.