Type constructor with new size

[Eppason via Digitalmars-d-learn]

How can I create a new type NT from type T that such that NT is 
compatible with T when reduced to the size of T, but has size n?

Another way to see it is that I would like to construct a type at 
compile time that has the same layout as another type but padded 
exactly by n - T.sizeof bytes. It would be nice if the new type 
is implicitly convertible to T.

This should be automatic and exact without overhead(pure, if you 
will).