If I were looking for a stiffening agent arrowroot or cornstarch would be the ones I'd think of first. I have used both and they both work well.
I found this about using arrowroot or cornstarch:
Both arrowroot powder and cornstarch are starch thickeners, powders used to give body to pie fillings, puddings, gravies and sauces without adding their own flavor. But they each have their pros and cons.
Arrowroot is a good thickener for an acidic liquid. (Cornstarch loses potency in acids.) Arrowroot is also a good choice if your sauce is mildly flavored. It holds up well to freezing, which cornstarch does not. When you're looking for a high-gloss finish, use arrowroot.
On the downside, it's more expensive than cornstarch, and it's not a good choice for sauces or gravies with dairy products: They turn slimy.
To thicken with arrowroot powder: Mix it with an equal amount of cold water and then whisk it into your hot liquid for about 30 seconds.
To thicken with cornstarch: Mix it with an equal amount of cold water before whisking the slurry into hot liquid. Simmer the liquid, whisking constantly, for about 1 minute before it thickens.
Agar agar could also be used to thicken your pudding. It's used as a gelatine substitute for vegans and you can also use it when making puddings from tofu.
I found this excerpt from a favorite go to book of recipes for vegan sweets and desserts that might help you figure out what to use:
Hope this helps and hope you make a yummy pie!