I started with two identical plastic soda bottles to make sure that they would match in size. I cut the necks of the bottles off using a small hacksaw making sure that I kept the large flange that would be used to glue the two necks together later.
Step 1: Cut off the necks
Once I had cut off the neck of each bottle I used a flat file to smooth down the rough saw marks and left over plastic. The idea is to remove all the rough edges until you have a flat surface level with the large flange as this will be where you will glue the two pieces together.