A flat plate with pins that locate into the bolt holes on the manifold and holes (with guiding barrels down to the head preferably) that allow bolts through the plate to meet with the mounting holes in the head, clamping the manifold between the plate and the head when the bolts are tightened down.
That's what I suggested - make the adaptor clampy thing in two parts that bolt together, and then sandwich the mounting flange of the plastic manifold onto the head.
Dunno, why?
Here's a first pass at Danstuff's idea:
A flat plate with pins that locate into the bolt holes on the manifold and holes (with guiding barrels down to the head preferably) that allow bolts through the plate to meet with the mounting holes in the head, clamping the manifold between the plate and the head when the bolts are tightened down.