Make and prepare cake according to the instructions on the box. Use a 9x13 baking pan. Cook for recommended time. Use a toothpick to test center to make sure it comes out dry and clean. You want the cake cooked through.
When the cake is done, take out of the oven and use the end of a wooden spoon to poke holes all over it. You want larger holes, not small ones, so that the peach mixture soaks into all of the cake. Let the cake cool while you prepare the peach filling.
Prepare the peach filling : take the large chunks of peeled peaches (4 peaches) and place into a food processor or a blender and blend up. I like to leave some larger chunks and some pureed peaches. Blend/chop to your liking.
Add the peach puree, sugar, and lemon juice to a saucepan over medium-high heat. Stir and let it come to a low boil so it can thicken slightly and the sugar can dissolve. This only takes about 3-4 minutes. Once it's slightly thickened remove from heat and pour over the cake making sure the holes are filled.
Cover and let refrigerate for 2 hours.
Prepare the whipped topping by beating the cream cheese and granulated sugar into a larger mixing bowl. In a separate bowl, or a stand mixer, beat the heavy whipping cream and powdered sugar until stiff peaks form. This will take about 5 minutes.
Add the whipped cream into the cream cheese mixture and stir to combine. Frost the cake, cover, and let refrigerate for at least 4 hours. When you're ready to serve, top the cake or each slice with fresh peach chunks.
Once you puree/chop the peaches, you should end up with about 2 1/2 cups of "peach juice" to pour into the holes in the cake.
If your peaches are really sweet then you may only need to add 1/3 cup sugar to them. Taste the peach puree and adjust the sugar depending on the sweetness. I added a little over 1/3 cup sugar to mine.
If you don't want to whip your own cream, use a 8 oz tub of Cool Whip in place of the heavy whipping cream and powdered sugar.