From what I have read of this post, Latin doesn't really use nouns as adjectives in the way that English does, although that post mentions you can use a noun in genitive case to achieve this. I am translating "Crater Lake" as the name of a location, and came up with the following based on that advice: Lacus Crateris.
Does that make sense?
On the other hand, it is perfectly classical Latin to use a proper noun and a common noun in apposition as in “urbs Roma”. If you want to take “Crater” as the English name of the lake you can write “lacus Crater”.
That sounds perfectly fine to me. You are correct about the genitive. An alternative could be Lacus Craterae; for both crater and cratera exist, with the same meaning, which includes the drinking-vessel and a volcanic crater.
It is conceivable that the Romans might have used an adjective, like ?craterius or similar, but I have not found actual use of this.
P.S. A famous crater lake is Lacus Avernus, where the entrance to my world is said to lie.
