Is there a way to thicken commercial boiled custard to use for a trifle?

You can also use gelatin. How much gelatin depends on how thick you want the custard.

I would probably try cornstarch--make a slurry with a Tbsp. or 2 of the cornstarch with some cold liquid, then bring the rest of the custard to a low simmer and slowly add the cornstarch mixture until it's thick enough for you (remembering that it will firm up as it cools). Good luck!