Mine hardly ever fit. I used to Put a match under it. But don't have matches anymore..
So the bit inside a biro that holds the ink, just cut an inch of the end where there isn't any ink.
Unscre the fin put it under then do it up as normal.
Really you can use anything, I often just grab a stick from the gutter and brake it up and use that.
If it moves sideways, you can put some tape on the side.
And I'm really impressed you uploaded a photo 2