There are two possible ways to create the plastic shell of your swimming robot. One is to use a 3D printer to print a simple model. Here is a link to such a fish robot: http://www.thingiverse.com/thing:305039.

If you don't have access to a 3D printer, you can also cut the model out of quarter inch acrylic, it's available at many home improvement stores. For this, get two squares of 12 by 12 inch acrylic. Now cut out the fish using this pattern:

You can choose different dimensions if you like; these were chosen to make sure the fish could hold a Raspberry Pi 3, a Raspberry Pi camera, an IMU unit, a servo controller, and a battery.

Now you'll need some hardware to bolt the two pieces together. Any smaller size screw will work; make sure it is slightly longer than 1.25 inches, 1.5 inches should work. Also, purchase some locking nuts as these will lock in place and won't become loose over time. These are also easy to find at most home hardware stores. You can drill...