Modern Vespa

This is probably a foolish question but... I'm considering getting a soft duffel-style tail bag to use with my Piaggio Liberty 150 - but the built-in rear rack is so tiny as to be almost useless. So I'm considering adding a slightly larger rack 'adapter' plate - which would then provide a larger 'platform' onto which I could strap my duffel. It looks like the generic GIVI plate is big enough - but I'm wondering if it's truly easy to attach to the Liberty? Any and all thoughts would be appreciated.

It's been 2 years since I sold my Liberty 150, so research this or wait until someone confirms/corrects me.
I think the Givi SR5611 adapter is required to attached the mounting plate. With that adapter, you should be able to mount the pictured mounting plate or even adapt something more to your liking.

Good suggestion, thank you! I'll look into it

Pop that plastic Piaggio insert off - and you'll get a good look at what you gave to deal with... hole placement, etc.
No experience with other than OEM Piaggio accessories.
O.S.

I put a Shad box on the back of my old Liberty. If you pop the cover off the rack on the Liberty you'll find three holes to which you can bolt anything. I made a metal plate to allow the box to mount further back. The same plate would also allow you to strap your bag directly to it.

I too put a Shad adapter plate on there for a Shad box. It looks nearly identical to the Givi. Very easy to install - you have to drill 3 holes in the factory plastic cover (it comes with a template) and then it all bolts together in an easy fashion. I do think that this would give you the extra space you need to bungee or otherwise attach something on there. It's silly that they didn't include a proper little luggage rack on there from the start.

Here is a picture of the installed mounting plate.

Thanks, much appreciated, this is quite helpful! Thanks for the suggestion - and THANKS for the photo of it installed, too! This is almost exactly what I had in mind

I installed the Shad plate and use a SH42 top box. The Shad was very easy to install, just some simple drilling, and I can't live without my top box now.

I wound up sourcing a 2-piece Piaggio Liberty mounting kit (a metal plate & a hard plastic rack) via a European eBay seller. The plastic rack part fit my Liberty after a bit of adjusting, but the metal plate didn't (it turns out the dimensions of the plate's triangular holes are slightly different for European-spec vs U.S.-spec Liberties). But the hard plastic rack fits well. I'm currently using a Rhinowalk soft bag (which seems to be an expensive though well-made copy of the cool MotoFizz bags that some folks liked years ago). The bag can be easily removed and has a nifty shoulder-strap for carrying.

You might consider attaching another "plate" on top of the plastic, either aluminum or wood, to extend side to side support for the bag.

Otherwise, this looks to be a great solution!