I used plastic corner guards (L-shaped plastic pieces from Home Depot) and siliconed them into place in the tank to make a slot to slide a divider into, then used egg crate as the divider, but that may be too large for your needs. You could also use a piece of the plastic mesh from a craft store that has smaller openings (like the stuff that is used for needlepoint).
My long term issue with suction cups is that I haven't found any that don't deteriorate, or become rigid, over time. Eventually they fail, and so does the divider.
Be careful with trying to wedge something in between the glass using friction. I did that a long time ago, only to find out that the pressure on the glass created a crack and I ended up with 10 gallons of water on the floor.