Preserving Plums

I picked almost all the remaining plums on my trees this week.   I made more plum butter, plum jam flavored with orange liqueur, and plum pie filling.  The plum pie filling is new to me.  Won't publish a recipe until I make a test pie with a jar sometime this fall.  But I did use some leftover filling in a plum "crumble" and liked the cardamom and almond flavors I used.   Regular cornstarch, tapioca, flour, and other usual thickeners for fruit pies don't stand up to canning.  The USDA recommends a modified cornstarch which home canners can buy online under the brand name "Clear Jel."