Glass jars are much better than plastic for use with alcohol as they will not degrade and leach chemicals into your sample. On the down side they do break, but with care it could last you a lifetime. The stable bottom keeps it from falling over and the narrow neck means you use less juice (try filling it 2/3 full to start) and it dampens movement there is not all that wavy back and forth.
NOTE: 12″ is too short for professional hydrometers and 15/16″ is too narrow for brewery grade hydrometers. Works well with all standard proof and triple scale hydrometers. They will not bottom out at any normal reading.