Was looking online for "plans" but have had a hard time finding any. Essentially map out the space you want to have as your overflow with the space for exit and return bulkheads. Once you have the measurements (length and height of the individual pieces), you need to get some acrylic. Black acrylic seems to be a favorite. You can buy this online. I would use something around 1/4 an inch thick.
Once receiving, you need to cut the acrylic down to size into rectangles that would fit together to form the dimensions you were looking for. Draw out the fins along the top, more or less, the water flow will differ. Most fins tend to be roughly less than 1/4 an inch. drill out holes roughly this size and then sand/cut out the rest.
Silicone together and in place. Just a warning, after making my own sump, acrylic does not bond well to glass. You have to use quite a bit of silicone. Might not look pristine, but it does work.
