The cheapest and easiest method to create an escape proof octopus tank that I have found involves replacing the canopy of a standard 10, 20 or 35 gal tank and cutting a sheet of glass (at least 1/8th inch) to just fit into the depression around the top. You have to measure this carefully and have the glass cut to precise specifications so that there are no gaps when it is in place. Glue handles or use suction cups to help in lifting the glass for maintenance. Do not use plexiglass. It will warp and leave gaps through which the animal can escape. Large octopus may be strong enough to lift the lid, so I always place a heavy weight on top of the glass. We also use a lot of custom built all glass tanks for photography. With these I just make sure that the top edge of the tank is even all the way around so that a piece of glass placed on top forms a good seal with no gaps.
To provide access for circulation and filtration, I use a diamond drill to cut two holes about 2 inches from the top of the tank. The location of the holes will depend on where you want the water inflow and outflow tubing to be located. For a 20 gal, I use a 5/8th inch drill which allows me to pass 5/8th inch tubing through the holes and connect a canister filter to the tank. The simplest and easiest connection is to use a 3 inch length of tubing in the hole and force barbed elbows into the two ends of the tubing. This will stretch the diameter of the tubing so that it forms a tight seal. A more secure approach is to plumb through the openings with pvc bulkhead fittings or use threaded male and female connectors. This will require drilling a larger hole. Attach the canister inflow to a fine strainer in the tank (I cover it with a foam block.) and the outflow to a spray bar with very small holes.
I've kept hundreds of octopus ranging from blue-rings various pygmy species to bimacs and Pacific Day octopus in these types of tanks and have yet had one escape through the holes for the tubing although occasionally we forget the replace the weight on the glass top and strong octopuses have pushed up the glass and escaped. For robust animals like bimacs or Larger Pacific Striped Octopus, latches added to the top of the tank provide added security.