Exterior foam with thermalbuck and European windows- no wood to attach window flashing to! – GreenBuildingAdvisor

I have a post and beam barn with exterior foam only and I’m using Thermaltight as a WRB. It’s sealed to thermalbuck which rests on 4” of wood and extends out to cover the foam. Drutex upvc windows are installed. We have a rain screen of vertical then horizontal wood strapping as the siding will be vertical corrugated metal panels. 

My problem is the sills. The thermal buck is supposed to taper and I only have the windows in about 3” or a little less from the edge of the thermalbuck. But they are not really (in every window) shedding water out. 

I have Contega IQ for tape. 

Metal siding is purchased and I have J channel and sill flashing. 

How do I trim these windows? There is nothing for the metal to fasten into. Should I put some wood over the thermalbuck after the tape layer and use that to make the sill more angled? And to fasten the flashing? Is there a way to do this without fasteners? 

Please help!