Pretty amazing to me that one of the busiest threads on the Heavy Equipment Forums is about septic tanks. lol.
Anyway, some information if interested. For our area, "old" tanks (sometime in the 80's and back) were commonly around 1,000 gallon concrete with 4 concrete lids in some fashion similar to what emmett518 describes. They can all be square but also there were a lot with angled off end lids like in this picture.
Anyone's best bet if they want a riser in square lids or angled lids like this is to purchase one already poured for it, or pour your own. It's not that hard if that's what you want to do. Get you a piece of plywood and some 2x4"s. Form the lid to the size you want on the plywood. Then form a square in the middle of that rectangular the size of the riser you are going to put on it. Be SURE to check your riser and make sure it will cover the finished hole completely. Use reinforcing wire and good concrete. Rebar is good for awhile, but does have a more rapid corrosion rate causing expansion, cracks, and often premature failure.
This is a picture of an up to date septic tank lid from a 1,000 gallon tank currently approved for our area. Plastic handle that folds down. Less likely to snag it with a backhoe. How long will the plastic hold up? Time will tell...
And finally, this is a version of a modern "short" riser. Risers like this are typically poured in 1' and 2' risers. Then there are short ones like this to make up a possible depth you run into where you just need a little bit more height. Risers can be stacked within reason to give you the desired height. So a 1' and 2' stacked can be stacked to give you 3' etc. etc. It may not show the size difference but this lid is larger than the tank one above and the riser covers the tank hole with about 3" to spare all around on the inside. You'll want to seal around all riser connections or seams with a good sealant. There are some tar based like tapes that are about 1" in thickness that are used. Also, troweled cement can work.
Finally a tip. Make SURE that your tank lid is removed before installing your riser or that it will fit through the one you install if you elect to leave it on. And pull it out & set it back in lightly. Don't drive it in, step on it, try to make it tight. Why? It can be an S.O.B. to get them out! Especially 3' down!
