Probably this is 'tricky' in that I have never done it before. The pic shows my problem. The walls were 10cms out from Vertical (over 3.5m) the French Doors were fitted to about half the error (not by me). So I have to seal this gap which is 20mm (face of PB from wood frame) at the bottom and, well you can see the gap up top (3m). I have put copious foam in the gap and I was thinking to cut a strip of plasterboard and either slide it past the wood frame or abut it onto the face of the wood frame. Then, with both, fit a metal corner jointing lathe and use jointing plaster to make good - albeit I would have to cut it down a tad at the bottom.

I am pretty sure I can fit the plasterboard strips - a bit more foam (low expansion), some silicone gun grade foaming PU glue and perhaps 'delicately' a few PB screws PB to PB and maybe just 'in' to the frame. I could cover the inside face of the PB with foil tape for extra VP protection. My worry is to achieve a top finish against the wood and for it to last. I will sand and finish the wood frame before. With either method I need to get some acrylic sealant in there after jointing or for sure there will be cracks. So all in all a bit of a pain! Am I missing something? Is there a better way? Would you abut or slide past? Many thanks.

Perhaps I should add that the loose fitted single pane was just siliconed in to help with airtightness a bit until I got around to renovating the room. I will rout out and fit DG and yes the doors are triangular at the top with the suspended ceiling following the angle, apart from where it had to go horizontal on the left to cover all my Solar and PV pipes/conduit and MVHR etc.

Oh well at least it is not just me. I have thought about the wood solution and it is unquestionably more work, nigh on impossible to get a convincing colour match without going very dark (for me anyway), costs more, and then I end up with a less modern look and very little room above the architrave for curtain pole - it'll look tight enough anyway on the top left corner (shown above).

So that's it then - I am off to cut back the PB. The metal corner beading/lathe is very strong, perhaps you don't use it in the UK because it won't crack. I'll post a pic idc.