I have just finished making this 20 minute detailed video that shows you exactly how to build two different sized underwater viewers. One is 12" x 3" and the other is 24" x 4", but you can make them any size you like. These are ideal for gold prospecting, fishing (seeing what the fish are up to) and just for fun—kids love them.
A few people say that using ABS cement to glue these together was difficult because it dries so fast. If you do not have much experience with ABS pipe and cement, you can use silicone instead (and save a few dollars, too). If you do use silicone instead of ABS cement, give the inside of the coupling and outside of the pipe (only where the coupling covers the pipe) a light sand to help the silicone bond better and only use silicone on the outside of the pipe, not the inside of the coupling. This will prevent the pipe from pushing silicone inside the coupling and possibly onto the plastic lens.
In addition, after you put the coupling on the pipe, add some weight on top of it until the silicone sets up. This keeps it from sliding back (which it may do a tiny bit, I am not sure why, seems to happen with silicone). This is important as you want the pipe to slide tight up to the Lexan lens for a better seal and to keep the lens in place.
I rough cut the sheet with a jig saw and then finished with my belt sander with 80 grit. The belt sander will heat up the plastic a little, so after it is cool (15 seconds or so), take off the sanding burrs with a mill file. You can also use a bench scroll saw, coping saw to cut the rough shape (with protective coating on the jigsaw base to prevent scratching through the plastic coating on the Lexan). If you use a jig saw, use a very fine tooth or metal blade to prevent chipping the plastic. No belt sander? Cut out a little more precise and then finish with a mill file. It takes a little longer, but works.