Mars Bar crispy cake - help with quantities

[Ceolas]

I'm making a tray of this for a party tomorrow and have found the recipe in the archives.

My tray is a shallow oven tray 40x28cm.

What quantities should I use?

I have a recipe for this but it is for individual cakes - I am not sure how you would alter it for a tray!

Maybe double the quantities and fill the tray then use up any leftovers in individual cake cases?

The mars bar cake I make is 3 mars bars, 3 and a half oz of butter and 3 oz of rice krispies. Cover with 150g of plain chocolate. That makes up enough to fill a 9 inch square tin.