Amy:
Below is a recipe for strawberry cake filling I found on the web. I have not tried it but it does look yummy.

Filling:
Half envelope unflavored gelatin
2 Tablespoons lemon juice
1 Tablespoon boiling water
1/4 pound softened cream cheese
1/4 cup superfine sugar
1/2 teaspoon vanilla
1/2 cup whipping cream
4 cups strawberries, hulled and sliced thin
Filling: Sprinkle gelatin over
lemon juice, let stand for 1 minute
or until softened. Stir in boiling
water until gelatin dissolves.

In large bowl, beat together cream
cheese, sugar and vanilla until
smooth. Beat in dissolved gelatin.
Whip cream, fold into cream cheese
mixture. Spoon evenly over cake and
arrange strawberries over filling.
Refrigerate for 1 hour until set before frosting remainder
of cake.
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Hint: Amy, I would make this a thin layer so your cake layers do not slide apart. Also, keep the entire cake refrigerated until serving.
And, dental floss works great for slicing the top off a sheet cake (or off a cake that rises too high in the middle.)
I hope this helps you.