Hi Wendi! Instant clear jel can help firm up a pastry cream a bit. If you're looking to add it in addition to the corn starch, I'd try sifting a teaspoon in at the end of the process after your butter is added. It won't be quite as firm and sliceable as a gelatin thickened pastry cream but a little stiffer than just corn starch. 

 We haven't tried using it in place of the corn starch but it should likely work, especially if you cut it with the sugar so it doesn't clump.   Weeping would likely occur around day 4 or 5 of storage.  Happy baking!

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