I agree - it looks like the Vader action figure case. As I see it there may be 2 possible answers.
1 - it is a moulded 'buck' piece used to produce the original cases, at Kenner (in the US) or Palitoy (in the UK), in which case it could be worth into the £1000's.
2 - it is a fan produced piece moulded from the Vader case, in which case it's a curiosity but not worth a great deal.
For further information I recommend you post it on one of the big Facebook groups - The Admiral's Snackbar would be a good one as it's specifically for rare and unusual items and many of the top collectors in the hobby are active there.
Good luck with it - I'm intrigued to find out what you have there!