First, a microwave oven can work in a pinch....but, if you want to build one for larger items, it's not that difficult.

Take a shed, 8'x10' and drive 8' ground rods in the ground around it, starting at the corners and then between them every 2'. Then get #6 ground wire and line the walls and ceiling with it, spacing the wire approx. 2"-3" apart. Make all the wire grounded together and attach it to the ground rods on the exterior, take the wire and attach it to all the rods around the exterior of the shed.

After that, get 4'x 8' sheets of 4" foam insulted plywood and cover the interior walls & ceiling. Seal the seams with fire/heat electrical puddy and tape over with foil heat tape.

I have many skills....but, I'm here in the US....would love to come to the down under! But, my wife would never entertain the thought.

Good luck my friend!