Well, you have a number of options. It's true that it's hard to have completely NOTHING entering the tank from the top...at the least you might have wires from powerheads or grounding probes entering there...I don't know if there are methods to route those in from the side, never having had a predrilled tank. Your sump return might produce enough water flow that you don't need supplemental powerheads...depends on what else you intend to keep in there, like perhaps corals.
Sealing the lid is easy if you've got nothing coming in (or just one or two wires.) But you're right, you've got to be able to still get in there easily. You don't silicone it shut or anything...rather, you just make sure that the lid has a very tight fit, and then you cobble together some means to make sure that lid can't be lifted from the inside. Weights help, but octos are stronger than you think, and often can overcome heavy weight. Duct-taping it down is not unheard of, many reports here that it works, and it's cheap. I would love to custom-build a tank with some sort of slot built into the top edges of the tank...I'd slide a plexiglass lid into the slots from the side, so it'd be impossible to push off, and lock it down with a clasp at the end so it couldn't be pushed sideways. Anyway...if you can go pre-drilled and use a sump, then you can contemplate how to lock the lid down later.
If you can't find a predrilled tank, a good store could drill it for you, or you could even drill it yourself, though that also means you need to know where and how to drill and how to affix the necessary plumbing bulkheads...I certainly don't know how. MAJOR WARNINGS!!! Drilling holes in glass is tricky, many stores can't or won't do it. All too often, the glass cracks when they try. Worse, tank glass is sometimes tempered. Tempered glass does NOT take drilling well, and most often shatters into a million tiny pieces like automotive safety glass. Obviously, if you can find someone qualified who can drill for you, your tank needs to be empty at the time!
I avoided doing a sump at first because I thought they were complicated and I was intimidated. That was a mistake...they actually make owning a tank much simpler, thus I've since installed a sump. Since my tank isn't drilled, I had to compromise...I'm using a hang-on-tank overflow/siphon system. They're readily available online...mine's from Lifereef. It works well, but it has a few drawbacks to a "real" sump. It gurgles more than usual, and it is hang-on, so although I was able to relocate my other hang-on equipment to the sump, I do still have at least one piece of major equipment up top. It makes octo-proofing much harder. I'll probably be able to get by with some precise cutting of plexiglass, some duct tape, and some siliconed netting, but it would have been much easier on me to do it right from the start.
Of course, this is more expensive, too. Long-run it's better, but short-run, when you're on a tight budget (which I was,) you make do with what you can. If you can't go with a sump early on, I'd recommend just going sumpless and then later, when you think about upgrading, getting a new tank or a hang-on sump overflow like this. At least whatever you use for a sump can be dirt cheap--I'm using an unusually sturdy $15 plastic tub (have to be careful there, some tubs can crack.) Some stores offer very nice tank packages with tank, stand, plumbing, and a fancy sump that's rather pricey but offers some nice filtration features. It kinda depends on your budget and your personal preferences. Don't feel bad about what you do get...you can't really go "wrong," only "more right," and until you get some time under your belt you may not know what "more right" is for you personally yet.
rusty
...Hmm, but is it really possible to have virtually "no-hanging" apparatus on the tank? If the top was to be sealed, what about cleaning the skimmer and insides of the tank when the need arises?
...If nothing goes wrong, I'll be getting my tank tomorrow...Hmm, but what if my tank isn't pre-drilled for a sump? Where will I place it then?
But sadly, I won't be able to get an octo afterall due to unforeseen circumstances like being disallowed to keep one 
Still, I'm sure that all the advice I've got will be extremely helpful in the future...So I guess I'll have to wait till then. Do share your experiences with me though, haha, it's an advantage to be well-prepared before going to "war" 
