I have just this minute finished icing a 8" x 13" cake. It wasn't as deep as yours prob about half that. You could use your usual quantities and if too thin, bake another? The recipe I used was for madeira cake, which was a lot firmer and easier to ice.
Cream marg and sugar together till light and fluffy In seperate bowl, lightly beat eggs and milk Mix flour and baking powder together an d gradually beat into creamed mixture alternating with egg and milk mixture Spoon into linied tin and bake at 160*/325f/gas mark 3 for anywhere between 30mins - 55 mins (mine took 55mins) until cake springs back when pressed lightly