In contrast to others, I would recommend the slim shell. My Glowlight 3 has that slim fit cover and it is big and bulky. My Glowlight Plus has the slim shell and it is much lighter weight.
Fintie only has a couple of slim shells left for the Clara and the designs don't appeal. So I bought this one:
Ratesell Premium Folio Smart-Shell
I think this one looks nice as well, but I won't bother replacing the one I already have:
MoKo Kobo Clara HD Case, Premium Ultra Compact Protective Sleep Wake Up Slim Lightweight Cover Case