If the shells are only sticking up a little bit, I would dome another layer of resin. For me anyways, it would be easier to do that rather than deal with drips that come around the edge from trying to get cast while back in the mold.
You could also run a piece of masking tape around the edge and pour another layer of resin. Once the resin has cured, you can peel away the tape. You will need to finish the edge, however, as there will be a matte finish. You can see what I’m talking about in this blog post: https://www.resinobsession.com/resin-resin-resin/resin-casting-with-backless-bezels