I don't think any of my zigzaggers work with the cover plate - they all rely on the feed dogs to move the fabric forward, they only do the side to side.
I've got at least 5 different zigzaggers that work with a 15-91 though all of them are at least 60 years old. I'm surprised there is not a "new" one out there to fit a low shank machine. Though I suppose if the seller looked up a List, the 15-91 is probably not on there.
Also, New is usually very expensive. I'm always shocked to see the "new" ruffler sell for $35 when you can pick up the almost exact thing (vintage) for under $10 on eBay, and it will come with several other attachments too. Shipping should not be more than $10 so you are saving quite a bit.
From what I can tell with rufflers, once they went to the five slot variation, they've stayed about the same since.