Cockney rhyming slang typically uses two words to replace a single word, where the second word rhymes with the original word.
While "goodnight" itself doesn't have a perfect rhyming slang match, it's important to remember that cockney slang is constantly evolving and there might be regional variations or less common phrases that are used.
If you're looking for a slangy way to say goodnight, you might use phrases like:
* "See you later, alligator!" (which is more playful and informal)
* "Catch you tomorrow" (which is also informal)
* "Sweet dreams" (which is more common in general)
Ultimately, the best way to communicate is to use language that feels natural and appropriate for the situation and your audience.