kerb insulation Flat roof lights - insulation of the kerb

[Nevoriana]

I'm detailing the up-stand (kerb) construction for a couple of fixed glazing roof lights in flat warm roof.  I've done a thorough internet search for design detail information - particularly for the correct positioning of the vapour control layer (VCL) around the roof light kerb.  Roof light manufacturer websites are noticeably unhelpful, mostly ignoring the need for any sort of VCL.  I'm constructing the kerb from resin bonded exterior grade plywood (WPB), and the insulation will be tissue faced PIR.  I can't see any way of fitting the VCL on the warm side of the insulation without building the kerb in two parts.  An inner plywood skin is clearly needed to allow the fitting of the VCL to its vertical outer face, as well as to the inner face of the insulation.
 

Latest best practice advice is to avoid any timber between the insulation and the weather proof membrane, so the question that arises Is how to continue with a reasonably strong and rigid box to support for the significant weight of the glazing elements, but with minimal penetration of either membrane.  Is preservation treated WPB appropriate?  I'll possibly need to post a drawing here, so my current thinking can be appreciated - if anyone feels it would help understand what I'm on about.

 

Does anyone have any experience of this tricky situation?