The tanks we have built for the local zoos and museums here have sealed tops, with one access panel that is clamped down. All of the filtration is from a sump located below the tank, attached with sealed in/out piping. While this is somewhat $$$blingbling, it does work quite well...some octos don't ever seem to want to crawl out of tanks, and some are little houdinis...guess it depends on the personality!
Our octi tank (for P. cordiformis) has perspex slides between the top of the tank and the ceiling and walls. We tried the covers directly over but the octi suckered on under it and flexed it til it snapped!! All of our drains have covers that just sit it the drains. So far the octi hasn't bothered with them. But if it does (& this has happened in the past) we'll glue them into place!
With the small octi (O. warringa) we have a flat perpex lid on the tank and plankton mess over the drain. That seems to keep them in!