I think your timeframe is fine. Even if you only use a crumbcoat of buttercream icing. I like to use the apricot glaze, but sugar syrup is cheaper and works well too.
Actually a lot of folks are under that impression about the pudding cups. Jeanne G from the Wilton site contacted several of the manufacturer's and they were horrified that folks thought these puddings could be kept at room temperature AFTER they were opened. Not so, they need refridgeration.
For a filling, they are one of the worse ones for separating or absorbing back into the cake. They are fine for a day or so refridgerated, but actually a cooked pudding filling holds up much better, maintaining its thickness and texture and not absorbing back into the cake. Whenever you do use a filling like this, always use a stiff buttercream icing dam, in about 1/4 inch from the outer rim of the cake. The easiest way to do this is to just use your icing bag with the coupler inside - it makes about a 3/4 inch thick bead which is perfect. Always fill lower than the dam. Replace top layer and press down slightly.
I think you are best off just filling this cake with buttercream. The hour in the car with the a/c will make the cake hold up fine.
Here is a simple cooked pudding filling.
Vanilla Cream Filling
To scald milk is to make it hot, but not to the point of boiling it. It has reached the scalding point when bubbles form a ring against the side of the pan but don't appear on the surface ot the liquid.
This can be made on the top of a double boiler with the bottom section filled with boiled water or directly on the stove in a small heavy bottomed saucepan.
In a small saucepan over medium low heat, scald : 1 cup milk - for best results use whole milk.
Combine in a small saucepan:
3 tbsp. all-purpose flour
1/4 cup granulated sugar
pinch of salt
Very gradually add your scalded milk and cook over medium heat, whisking constantly, until thickened. Cover and cook 2 minutes stirring occasionally. Stir a small amount of the hot mixture into :
1 egg, slightly beaten with a fork
Blend this mixture into the hot mixture and cook for 1 minute longer, stirring constantly. Remove from heat and blend in:
1 tbsp. butter
1/2 tsp. vanilla.
Let cook. Makes 1 cup filling, enough to fill an 8-10 inch cake.
Almond Cream - Follow basic recipe but substitute 1/4 tsp. almond extract for the vanilla and add 1/3 cup slivered blanched almonds at the end.
Banana Cream - follow vanilla cream instructions, but just before serving add 1 banana sliced
Coconut Cream - Follow recipe but add 1/2 cup desiccated coconut at the end.
Coffee Cream - Follow vanila instructions, but add 2 tsp. instant coffee powder to the scalded milk.
Butterscotch Cream - Substitute 1/2 cup lightly packed brown sugar for the white sugar and follow other instructions.
Chocolate Cream - Follow vanilla instructions but substitute 1/2 cup granulated sugar and 1/4 cup cocoa for the sugar and salt called for in the recipe.
Always let fillings cool down to room temperature before filling a cake. Refridgerate after filling.
Hugs Squirrelly Cakes