Okay. Here's how the story ends. I've thrown up my hands and surrendered. The windscreen will not fit no matter how hard I try. So I take it to my local bike shop, Scooterville, and arrange for them to do the installation. They can do it, they say, but . . . they'll have to modify the turn signal brackets. Otherwise, the mounting bracket cannot be attached. Oh, and they'll have to use a different spacer for the mirror; the one supplied by Scooterworks won't fit. The good news is that it'll only cost me $65.
But the bottom line is this: (1) Scooterworks advertises this particular screen as appropriate for a 2009 People 150; (2) although the Scooterworks website indicates that you need to relocate turn signals if you're installing the Prima Large windscreen, there is no such information on the listing for the Extra Large windscreen; (3) Scooterworks provides no installation instructions with this windscreen; finally, (4) the Scooterworks Sales Director swore up and down that this screen fits this bike--he even said he put it on his own bike, with no problems at all--either he was making that up or he was flat-out lying.
So, if you're in the market for a windscreen, I have two suggestions: first, do not, under any circumstances, buy a Prima--it'll only cause you heartburn; second, be careful about doing business with Scooterworks.