While I'm a little taller than you, the Child Bride is just a bit over 150mm/6" shorter than me too, so the height difference we have is similar to yours, and she did have a similar complaint back in the early days of our Spyder Ryding, plus there was the very concerning (to her!) issue of exhaust fumes back there and the amount of rain & slush that got picked up behind the RT & dumped down the back of her neck - she 
REALLY didn't appreciate riding back there in the (light) snow that we get here in Oz because of that!! But, after I'd spent a little time riding around with hundreds of 75mm/3" long wool tassles taped all over the RT in order to see what the air flow was actually doing as it passed over & around the tupperware, we came up with a solution!!  
On the pre-2020 RT's like yours & mine, you might want to consider adding a 'Whale Tail' type luggage rack to help smooth out the air flow around and behind the Spyder. The raised/fixed rear trunk our RT's have causes 
a lot of turbulence and buffeting back there, and a horizontal rack on the trunk lid helps iron a lot of that out, reducing the buffeting on your pillion passenger and virtually eliminating sucking up all the exhaust fumes & the water/ice/slush that then gets dumped down the back of your pillion passenger's neck when you're riding in the wet! Check out the Rivco Extendable Rack:
Here's the Chrome Rack: 
https://www.rivcoproducts.com/shop/can-am-spyder/racks/ca040-chrome/
Here's the Black Rack again: 
https://www.rivcoproducts.com/shop/can-am-spyder/racks/ca040bk-black/
And here's one of my posts in another thread with a couple of (dark/inside) pics of one on the back of my 2013 Spyder: 
	
	You mean something like the rack in the pic below?? (Apologies that they're so dark, but hey, it was night-time! 

 )
https://www.rivcoproducts.com/shop/can-am-spyder/racks/ca040bk-black/
That Rivco rack not only works as an extendable luggage rack, but it also helps to smooth out the air turbulence behind the Spyder that can lift up a lot of road spray & dirt etc as you ride & dump it down your pillion passenger's back; and it'll also reduce if not eliminate any gas/exhaust fumes being swirled up & into the 'cockpit area' behind the windshield too! 

hyea:

 - 

 
 
Aside from that, you could do a search on a 'Laminar Lip' extension for the top of the windscreen - the right one of them will help lift the height of the airflow over the top of the Spyder, hopefully enough to leave your pillion in the larger bubble of clear air behind the windscreen. Altho I think that, given your shorter heights and our experience, it's probably more likely the previously mentioned turbulence due to the RT's rear trunk that's causing your wife to be blown around a bit. 
 
 
Good Luck! 
