A boat tail is the tapering of a rocket's body as it approached the end, to reduce aerodynamic drag. The optimal angle and length of your boat tail depends on the rocket's flight profile and other characteristics, so it is best to use software like OpenRocket to auto-optimize the design.

It is important to note that you should not have the motor recessed too far inside of the boat tail. While sometimes the boat tail can be used to thread in a retaining plate (like in the image below), recessing too far will cause re-compression of the exhaust plume and reduce motor effectiveness by as much as 80%. Generally, you should not have the motor more than 40% of the motor's diameter, i.e. a 100 millimeters motor casing should only be recessed about 35-40 millimeters. Some research has shown that slightly recessing the motor into the boat tail can increase exhaust pressure, and therefore thrust, but in general, it is inconclusive. 

Example: The machined boat tail of the RIT Launch Initiative "Water Bearer" rocket.

Material Selection

Usually, you want to make your boat tail as light as possible. Most rockets require more weight towards the nosecone, so a metal boat tail would throw off the CG and cause unstable flight. 

3D Printing is a good way to make a boat tail that is lightweight, although a material like PETG should be used to prevent the motor from melting it.

If you are planning on using the boat tail to also retain the motor like the example above, aluminum should be used. 3D printed or plastic parts cannot generally provide the strength and temperature resistance required to retain the motor.