Your first attempt is almost correct, you need to move que after the object of the verb (and du is not correct here btw).
Je n’ai jamais l’opportunité de le voir que pendant les vacances.
The second one is a little trickier because plus can mean two things, and plus jamais means never again (not never more). The second one should be:
Il ne sera jamais qu'un ami. (never more is implied, just like in English)
Or alternatively:
Il ne sera jamais plus qu'un ami (here jamais means never: "He'll never be more than a friend".)
(I used bold because the s is not silent)
The thing is, jamais in the sense of ever is rare and very literary, it's almost never used in spoken French. So it's correct for an essay (although your teacher might tell you the same thing) but I wouldn't use it for colloquial language.
For the first sentence you can juste drop it:
Je ne peux le voir que pendant les vancances
Je n'ai l'occasion de le voir que pendant les vacances