How to construct wildcard type in Scala macro?

I am unable to find out how to programatically construct existential types in Scala macros.

For example, let's assume that I have a ClassSymbol that represents a class C[T] that has one type parameter.

Now, how do I programatically construct the type C[_ <: java.lang.Number] ?

In particular, I have no idea how to use the ExistentialType constructor object. Looking at its signature:

def apply(quantified: List[Symbol], underlying: Type): ExistentialType

What do I pass as quantified?