do you mean you cannot heat the custard to make it? Then you will have to use the so called "instant" puddings, but frankly, they are not great. THey have to stay cold, and the dessert has to be eaten soon after preparing (it wont hold up).
Choosing a dessert depends on many factors, but I would avoid "instant" puddings unless you can chill it well before serving, and you expect to eat it all when served. See this...
http://allrecipes.com//Recipe/chocolate-trifle/Detail.aspx