How do you thicken a cold filling?

The number of available thickeners is very large. Some work for high temperatures, others for low temperatures. For your filling I would use Iota Carageenan (Its used for thickening in many commercial ice cream products). It might be a bit difficult to find. Your second choice would be Xantham Gum.

When using these thickeners be sure to hydrate correctly and measure thickener weight as precisely as possible (to the gram). Between 0.5% and 1% of the liquid's weight is a safe bet but it will require some testing.