I recently saw on a forum that someone thinks your show only accepts transformers in metal boxes.

Our response to this gentleman sets out our position on this matter.

Any box which can satisfactorily encase the transformers and prevent physical contact with those parts which carry mains voltage is acceptable.

Whilst metal or ABS plastic are the more preferred options for home designed power supply enclosures, a plywood/MDF construction will also suffice. Consideration should have been taken for ensuring adequate ventilation either by free flow or fan assistance as the transformers can generate heat. Any holes provided for such ventilation should not allow direct contact with exposed live parts inside - i.e. cannot poke a screwdriver through and touch live parts. 

The other proviso we check for during set-up is that if there are metal parts (bolts) etc used in the mounting of the transformers to the cabinet chassis and they extend to the exterior of the casing then we would expect there to be a physical electrical bond to the common earth conductor. This is such that if a fault does present itself and provides an electrical path to earth potential by persons touching exposed metalwork, this leakage path would cause the Residual Current Device to operate and disconnect the mains supply. 

The same rules regarding bonding to earth would also apply to lighting installations which use mains voltage and metalwork supports unless the light fittings were specifically of the double insulated type.