I'm making a cake that everything can sit out after it's done, except the filling possibly.  The cake is going to have a RKT turtle on the top that is covered in fondant and have fondant legs & tail that will be lay beside him on the cake (I know it sounds weird but it's not when it's done).

Anyway, below is a recipe that I used for the lemon filling that they want and since there were egg yolks in it, I'm not sure I can let it sit out of the fridge over night.  It gets delivered tomorrow right after I get off of work, so I don't have time to do it when I get home.

• 3/4 cup sugar
• 2 tablespoons cornstarch
• 1/8 teaspoon salt
• 3/4 cup cold water
• 2 egg yolks slightly beaten
• 1 teaspoon lemon peel grated
• juice of a medium lemon
• 1 tablespoon butter or margarine

The egg yolks in lemon pie filling may be eliminated completely or replaced. When eliminating the egg yolks, add another tablespoon of cornstarch (or unflavored gelatin) with a little water (to make up for the yolk liquid) in the filling to make up for the loss of thickening power. Since egg yolks contribute not only flavor but color as well, you may want to add a drop or so of yellow food color to the filling. 

the boiling temp is 212 degrees the temp required to kill salmonella is 160. Sugar also acts as a preservative. I know I can use 1 cup of sugar and 4 egg whites in icing, heating to 160 degrees and it will be a nonhazardous food item. That's all I can tell you without having your food tested for PH and water levels.

AIf you're not married to that recipe, just google "shelf stable lemon filling" and try something new. Or just refrigerate the whole cake.