Screw the router router template to the speaker box, then shim it out a bit to allow for the gap between speaker box and baffle.
Remove the speaker - easily done by lifting it out through the removed top of the box - and put it exactly where it's going to be on the shelf.
Attach the baffle face to the soffit structure, clamber inside the soffit and draw around the outside of the template.
Remove the baffle from the soffit structure. The pencilled rectangle shows where the router template was. The black squares in each corner are pads of 4mm thick MLV, because that's what's going to be added under the whole baffle face.
Align the router template to the transcribed rectangle on the back of the baffle face then screw it in place.
Route out the speaker hole using the template bit.
Well, now it's done, and we take it upstairs praying it will fit...
And it does

Then repeat all that for the other speaker

Cheers,
Jennifer