Normally the home and removeable-media interfaces are enough for this sort of thing since they cover the areas of the filesystem the user typically has write access to outside of snaps. There is a process to follow for requesting classic confinement here: Process for reviewing classic confinement snaps and part of that process involves describing precisely why the snap requires classic confinement. If the home and removeable-media interfaces don’t work for you, can you describe how your snap operates and what sorts of things are denied in strict confinement?