Why not?

A plastic headbox, or at least a headbox with a significant portion of it made of plastic, would be lightweight, inexpensive and disposable.

We are not talking about some sort of FRP structure here, but a headbox whose main material of construction is a resin specifically designed for this purpose.  

Such a headbox would be able to easily be designed and constructed for laminar flow conditions, if desired, and streamlined corners and curves. 

We have been building headboxes nearly the same way for decades--weldments of plate carefully joined and polished.  It is expensive and may be unnecessary today.

Perhaps the first place to start is with a liner for an existing headbox known to have flow characteristic problems.

Such a headbox could still have a stainless steel machined slice, we are just talking about the body here.

Any thoughts?