Hey GBA. Is a gap at the back of a pre fab sill pan a big problem?

I got a Jamsill 4 9/16 for my vinyl slider (4 1/2″) that does not have a nailing fin. After thinking about it I went ahead and made my own nailing fin for better flashing.

Now the sill pan will have a gap at the back. Typically there’s a bead of caulk but couldn’t I accomplish the same with spray foam and trim it?

I could fab a new one with metal but prefer to use the Jamsill.

thanks!