As another reference point, I am using the 53mm short funnel on a La Spaziale S1 Vivaldi II. I also find the funnel sits loosely on top of the basket and the funnel is probably 1-2mm too small in diameter for a "snap fit" as shown in the OE slap shot video. If I tried the slap shot technique the funnel would certainly fall right off the top of the basket. It still works great for WDT, however.
I know OE makes a 51mm size, and at first I thought maybe I received the 51mm funnel instead of the 53mm, but from the commentary above it just seems to be more of a fitting issue across various baskets. Not sure what the likelihood is of OE shipping the wrong size. I may try Ton's suggestion to put tape on the funnel collar for a tighter fit.