The first order of business is your rope. The rope is just that, a rope, with discs tied on that fit into your pipe. You'll be needing a pipe very soon to put these discs in, and it's often easier to make the discs fit the pipe than the other way round so it might be an idea to source one of those first.
Size is crucial here. Our discs were 29.5mm in diameter because our pipe was 30mm. We tried larger and smaller sizes but these each had their own problems. Too large and the discs don't move fluidly in the pipe, so they can't just be a perfect fit. It's actually fine for a small amount of water to escape round the sides, in fact this lubricates the discs. If they are too small, however, more water escapes around the sides. This still works but you must move the rope through the pipe a lot faster to get it to work. As efficiency was our challenge we found 29.5mm, so half a millimeter smaller than the pipe's inner diameter, to be the perfect compromise.
Truth is you can make the discs out of a lot of things. We tried wood initially but it swells as it gets wet and there is a lot of friction with the pipe so we wouldn't recommend that. In the end we settled with acrylic. We cut the discs out on a laser cutter with a 5mm hole in the center for the rope to go through. 5mm is a good diameter for the sort of rope you should be using, just ordinary string gets really soggy and has a tensile strength that's worse than Heskey is at shooting on target. Thick enough rope also means you only need one knot to hold the discs in place as it won't slip through the hole. We used a mixture of 3 and 4mm thick acrylic, this was by accident though but it makes little difference because of something else we had to alter in the discs.
Just having acrylic discs is problematic for two reasons. The first is that they, while denser than water, don't sink fast enough to not get caught coming into the pipe as they are dragged in sideways as opposed to upwards from the bottom of the tank. The second problem is that in the pipe the discs can tilt, which stops them pushing as much water up.
To make the discs sink fast enough we added weight. We did this by gluing a washer on to the disc, trying to keep it central so as to not encourage further tilting in the pipe. This works well and washers are cheap too so that's handy.
The way to stop the discs tilting is to instead of having one disc each time, tie two right next to each other (one with a washer, one without - making a 'washer sandwich'). This makes the discs thick enough that the distance from the one edge to the other on the opposite side of the disc is larger than the diameter of the pipe so it cannot tilt very much at all. One thing that is important is to not glue the two discs together. This is as a more rigid overall piece gets caught a lot more easily. If you leave the two discs separate then they can move around a little bit as they should be tied on slightly loose. This massively reduces the chances of your discs jamming in the pipe or getting caught trying to get into it.
Before you tie the discs on I would recommend reading the next step as it shall heavily influence the spacing between the discs.
So to summarise, we have a washer sandwiched between two discs tied on to a rope at regular intervals.