Coregistration a partial func to a full struct

I have a full structural image and a functional image with the bottom half of the cerebellum cut off (i.e., the func image covers 95% of the brain).

When registering func to struct (or to standard), is there anything I should keep in mind? So far I’ve tried traditional FSL FLIRT and FNIRT, as well as BBR. Both outputs look okay, but not fantastic.