basically taking the part that will be pressed against the inside of the tank walls, adding some silicone to it (same as the sealant used to keep the seams of the tank together) and basically gluing the divider to the tank so that they can't push it away from the sides.
hopefully that made sense
as far as your bettas being able to see through, you can put lots of plants against the divider but if you look at the picture that shawnie posted as long as the divider is colored and not clear they shouldn't be able to see through it very well