You can make a slurry with cornstarch and water and mix it into the filling mixture with a whisk to thicken up the sauce. Whisk rapidly to avoid lumps forming. You can also temper the slurry by mixing some of the hot filling mixture separately with the slurry in a bowl on the side then putting it all back into the saucepan. This "tempers" the slurry and makes the temperatures of the two mixtures more similar so that the cornstarch doesn't cook in lumps before it has a chance to thicken the sauce.