I mail ordered a Piaggio Vespa OEM chrome rear rack and an OEM Vespa top box from Easyparts and installed it. Here's how with pictures.
Note: this DIY is accurate for the 2012 (and later) Vespa LX 125/150ie 3V. The rack is now complete as opposed to the earlier design where the rack was an addon to the existing grabrail. The latest LX grabrails that ship with the scooter no longer have holes for bolts. The old design is presumably discontinued.
Some preliminaries:
1. Remove pet carrier
2. Remove battery cover (one Phillips type screw), disconnect battery leads (-+/+-), and put the battery aside
3. Remove fuel filler cap
4. Remove five torx screws that hold the grey plastic panel in place (one of the torx screws is bigger)
5. Fuse box must be lifted off
6. Gently push the panel away to the side; there is a cable attached with a zip tie -- leave them in place