The easiest thing to do here is to purchase an old ammunition tin from an Army surplus store. The tin need not be very big a tin 1ftx4"x6" will do to start but you may want a bigger box later for larger engines. Some officer from the Police and trading standards/fire department are happy with the box as it is but others like to see the box lines with wood to protect the explosive inside from electrical sources of ignition or heat( the wood acts as an insulator). The Box should be locked with a strong padlock and be marked as with the Word "Explosives"