Go to lowes or homedepot and get a 1/10 of an inch sheet of clear plexiglass. Bring your inside measurements with you and they will cut it for you. The next step is to draw a grid with a sharpie on top of the protective film to use a guide. Use 1 inch spacing for the grid. Then take a small drill bit but large enough to let water flow through and drill the holes through the plexiglass, if it starts cracking the bit is too big.
To hold the divider in place, use 4 suction cups per side to hold the divider in place.
To hold the divider in place, use 4 suction cups per side to hold the divider in place.