I have an acrylic top that I bolted down to the top rim of my tank. I glued square acrylic rod to the underside of the top just over the "teeth" in my overflow, to fill the gap between the top of the teeth and the tank lid. The slits on my overflow are 1/8" wide, so I can't keep a really small octopus in there, but any octo with an eyeball larger than 1/8" should be fine. For a smaller one I would cover the top edge of the overflow with some open-celled foam (like the foam blocks used in AquacClear filters, that water flows through easily). You would need to be able to remove and rinse the foam periodically. If the foam clogs and blocks your overflow, your pump will try to put your whole sump into your tank (onto your floor).
Your live rock will be in the display right, not in the sump?
You only need to cycle the volume of your tank through your sump four or five times per hour, so a return pump that moves 150 - 250 GPH, at whatever head pressure you'll have, is what you should design for (A pump rated for 350 - 600 gph should be in the ball park). More than that is unnecessary, and requires your overflow to be larger. Use Koralias or a closed loop to get more flow in the tank.