About 30 gel beads is the correct quantity. I keep a bottled water bottle full of beads already soaking so, when they need refreshing, I can just pour out the dry beads and refill with wet beads.
I switched to clear beads - the pink was a prototype
Also, Home Depot sells precut industrial strength velcro discs (they label them as velcro coins), they aer the perfect size for mounting this in your case.