You need to read (or re-read) the installation guide for the Linea Boards Scyon - they are a 'system' approach and for proper installation the various accessories are supposed to be used - this includes the aluminium window adapter strips to give the correct finish.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.