Current octopus tank setup:
I used the back half of the glass lids to cover the back half of the top. I used window screen to cover the front half for ventilation and heat reduction. The screen is held in place with duct tape.
Since this tank isn't drilled I use HOB equipment; a refugium and skimmer. To fill in the gaps between the equipment and lids I used styrofoam cut to fit, wedged in tight, and taped into place with duct tape. All gaps, holes, and spaces where the octopus could climb through are also covered with duct tape.
The ugliness of this setup is why I would suggest using a drilled tank.