I used an extension of pvc pipe, about 8 inches, and an end cap. The pvc has lots of 3/8th inch holes in it. The downside is that might not work with a real small octopus, and it causes a little drag on the drain rate. It needs to be cleaned regularly but it's easy to clean. I will do this with my next tank. As a precaution though it would be wise to buy a "window screen repair kit" and cut pieces to fit whatever things you want further octo proof, and you can secure them in place with rubber bands or zip ties.
I used styrofoam to fill in the gaps around the top of my tank, and lots of duct tape.