DO read as much info as you can on hardware as I am NOT a hardware expert and tend to keep as simple a tank as I can get away with. I likely pay more attention to maintenance and keeping costs minimized than any other aspect. One place I don't tend to skimp is on the top so my acrylic bills have been high on even a cheap (as in used) tank. The tank opening you display is ideal for making an aesthetic top and you have a number of options. There is a stuck thread call "
HOW TO ...) that has a couple of topics listed and links to threads discussing the topics (ie the information is not in the stuck thread, it is only a cross reference). Both lids and drilling have links.
DIY Lids
Note any time you cut acrylic, SAND every surface that will come in contact with an animal or your hand or arm. Cut acrylic is very sharp and 15 minutes of sanding will help keep the red stuff in your body and the blue in the octo's.
My favorite method is to line the underside of each opening with thin acrylic strips, properly welded with acrylic cement. It will reduce your opening size so keep this in mind. I recommend attaching the strips around all 4 edges (to help prevent the lid from warping, not so much for support as 2" tabs would probably be enough. Diagonals at the corners might actually work well - I have not experimented - but could be a problem if they chip). The actual lid is sheet acrylic the same thickness as the top, cut to fit the opening (make a template BEFORE you add the supports and make one for each side, don't assume they are the same!) and when closed will be flush with the top of the tank. We drill the lids (using a peg board as a template to make nice evenly spaced rows) to allow heat escape and minimize warping caused by the dry top side and damp underside.
This kind of lid does not need a hinge but I prefer to have it open rather than having to remove it and find it again so we use a soft acrylic hing on the back. A hing may be more trouble than it is worth if you have a hood that interferes.
You will need some kind of handle since the top is flush with the tank. If you drill the tank, a simple (but ugly) solution is to thread a zip tie through two of the holes. If the lids are fairly large (a foot or more) then something rigid glued or taped (I have used automotive two side tape with some success but it eventually has to be replaced after a couple of years).
For our largest octo tank, I have a square 1" acrylic bar attached to the largest center lid that extends about 2" beyond the opening for ease of use.
You will need to lock the lids when you have an octopus in residence. I use an acrylic hasp latch and attach them in various ways (I also use two large marble book ends on one tank). The hasps work well with flat or 90 degree surfaces but not so well for uneven connections. Sadly I learned recently that you MUST USE the latches when you have an animal in the tank. We had been in the habit of not latching the covers on the large tank as we had never had a resident that showed interest in the air space above the water ... until we found Margay in another room, half way down the stairs. We did not find her alive.
Drilling
I have one bottom drilled tank and one low drilled tank and will never have another tank that is not side or back drilled. Leaks are fixable in side or back drills, you have to empty the tank if anything leaks underneath. Additionally, water can only empty to the bulk head (a stand pipe in a bottom drilled tank minimized the possibility of a total drain or at least slow it down to a drip). Ideally, if you have room to lower your water level about 2" from the top of the tank, this will help to keep the octopus from exploring ways to escape (but as noted above it is not a guarantee). Most tanks are drilled with less air space. Since this is a freshwater tank as it comes from the manufacturer you will also want to create a
weir around the bulkhead, again something easier to do with acrylic but silicone will work on both. Depending upon your pocketbook they have some nice new designs (I saw some in videos of this year's MACNA but have not seen them in the stores) that are easier to clean. The one in the linked photo is typical of a back or side drill installation. I recommend buying one over making one as the teeth are really, really hard to cleanly cut. Googling aquarium weirs or overflow boxes will give you some ideas (probably more than you want but
this is the one I would look into if I started a new tank today).
Speaking of water levels, in case I have not mentioned it, the amount of water that can drain from your tank with the power off is a consideration of how high the water is maintained in your sump so expect to fill it only about 2/3 of the way when you are determining your pump level for your skimmer. We mark our high level with a sticker that helps both to keep the evaporation minimized (you will have to top the tank off with fresh water on a regular basis in addition to more frequent water changes than you may be accustomed to) AND allow for power outage back drain (The last photo in
this post shows an example).