run-time stack-based allocation

[Gor Gyolchanyan]

I'm working on dynamic memory layout manager. Simply put, it will
allow one to create and use struct types at run-time.
Normally, you create a struct at compile-time type by specifying an
ordered list of fields, each with its own type (basically a size) and
name.
You then access those fields by calling a compile-time evaluated dot
operator, which computes the address of the specified field given the
address of the struct.
What I'm trying to make is precisely that, except at run-time.

My question is: what is the best way of allocating such a structure on
the stack? It will, of course, have a dynamically known size.

-- 
Bye,
Gor Gyolchanyan.