[PeterAllen]

Something I forgot to comment on previously. You have shown where you laid fiberglass over the 6mm gap you created and I can see that you correctly chamfered the two edges back but you have not indicated whether you also laid fiberglass of the reverse side. This is a very important process to avoid cracks appearing later on. You MUST sandwich the join not simply build it up from one side.

Here is an example of how I blocked off side vents. You can see how laying from one side will create a stress concentrate along the edge (photo 3). You need to dress and chamfer (if possible) and layer from the other side as well to alleviate this.