Try a circular shape with slightly more mixture at the edges so that it may be served by placing goodies in its centre depression. When cooked, turn the oven off, leave the oven door slightly ajar, and allow to cool slowly in the oven. This slow cooling works to prevent the loss of too much height. Serve cold, with whipped cream and fresh fruit pieces, strawberries and kiwi fruit for example. Cornflour is known in North America as cornstarch.
Castor sugar is known there as granulated sugar.
Make sure that you start off with a clean bowl and beat the egg whites well. A plastic bowl with scratches prevents the eggs from attaining a very light texture. Also ensure that the sugar is fully dissolved before going on to the next step. I have never had troubles from over-whipping. The final texture should be meringue on the inside with a thin crust on the outside.