A couple of months ago I stumbled upon a discussion board that talked about and showed a small, lightweight reusable container made out of the necks and lids of two plastic soda bottles. I haven't been able to find that same post ever since, but remember enough about it to want to give it a shot for myself.
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
I used a vice to hold the bottles still while I cut through them. If you don't have a vice you can still do the same thing by hand,but be very careful and take your time.
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.