There are many box sets and 3-in-1 covers in here mate:-

http://originaltrilogy.com/topic/An-Index-Thread-Box-Sets-3-in-1-Trilogy-covers-single-case-sets-useful-links/id/53673

Hopefully you’ll see something that catches the eye, though there are some some decent art/poster sites in that link too. It may also be worth having a look at Star Wars book box art or audio art for ideas and inspiration (and maybe other sci-fi content too).

Something like this, maybe:-

Good luck with it 😃