First of all, could, good, and cook don't have /u:/, they have /ʊ/, a completely different vowel.
This is /ʊ/: https://en.wikipedia.org/wiki/Near-close_near-back_rounded_vowel
This is /u/: https://en.wikipedia.org/wiki/Close_back_rounded_vowel
Your cuckoo sentence is pronounced:
/haʊ mɛni kʊkuːz kʊd ə gʊd kʊk kʊk ɪf ə gʊd kʊk kʊd kʊk kʊkuːz/
Secondly, those pronunciation-exercising sentences are called tongue twisters. Tongue twisters are phrases contrived to be difficult to pronounce as you are exposed to it, such as the famous "She sells sea-shells on the sea-shore."
Now, your direct question: "Make a tongue-twister for /n/ and /l/", here's what P.E. Dant said in your comments:
Nine nimble nobles nibbled nuts, might help with /n/. You can find thousands of tongue-twister sites, with entries sort by letter. Here is a page with several which are intended to help with /l/. Try a Google search for "tongue twisters for /l/ and /n/".
What is "tongue to a sentence" of N & L sound?
The point of using tongue twisters for pronunciation exercises is to be contrived to be hard to pronounce so that you can improve on them, and consequently the sounds the twister contains. I think you can make up your own, as you've already done.