Here we go then:
An A1 sheet of 3mm Foamboard cut to the right width, the total length of base and rear wall measured out and, where the two meet, i've scored lightly so it ebnds into an "L" shape.
Put in a seperate Foamboard former to break the rear wall up a bit and to also provide structral strength.
The grill there is going to be the main detail piece and is from Wilkensons and a snip at £1.69. I've started putting on greeblies and will be needing a ton in the bottom right-hand corner as thats where the company logo is cast on.