The opening looks almost triangular, but isn’t. Carefully join vertex to vertex, and reverse faces where necessary.
There were four triangles needed to completely close the hole visually.
I found I needed to draw over two or all three sides of some triangles to get them to close.
Speaking for myself, I find it hard to tell which faces are selected, and which reversed, with your choice of front and back face colours, but each to his or her own.
Certainly, modelling at 1000x scale helps - it would be impossibly difficult otherwise.
Patience needed, though, to fix the remaining holes.
PS. And when I use Solid Inspector 2 I find two problems with this fix.
I made it in the wrong context, which I could fix by cut to clipboard, open the group for editing, and paste in place.
There’s still a Surface Border issue on what is apparently one edge, but isn’t.
Why do you have two nearly identical groups, and which one are you trying to fix?
I’m sorry, but I don’t know of any simple way to fix the holes, other than painstakingly drawing edges to join all relevant vertices.
Do you have access to any other format of the model? STLs are always triangulated, and can be hard to fix, as these are.