The 'spoken version' is of course used a lot. I seem to remember from grammar classes way back when though that the 'for which slots ...' would be correct. We were always taught (Canada) never to end a sentence with a dangling preposition.

We in Ireland were taught the same as in Canada

I believe it is grammatically correct to write "for which." I agree with Canada & Ireland!
--from Indiana

My proposal: Have a look at "Mrs Thistlebottom's Hobgoblins", a hint Ghol gave as a follow-up on Dec. 8 (16.12h) in the 'correct translation?'-section concerning "stati". The URL is there.

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In standard American English, you aren't "supposed" to end a sentence with a preposistion, but of course it's widely done (even in official/academic sounding writing). The problem is that a lot of the time the "correct" version sounds awkward. There's a great example in the the Beavis and Butthead movie, where one of the FBI agents is trying to reformulate the sentence "we found the trailer that they were jacking off in", without ending it with a preposition. Of course jacking-off is really one word, but you get the point. So I think it's your call. There's also the old joke about Harvard (or Yale, whatever): A visitor to the Harvard campus stops a student, who is passing by, and asks him, "Can you tell me where the library is at?" To this the Harvard student snootily replies, "at Harvard we do not end sentences with prepositions." Asking again, the visitor says, "OK, can you tell me where the library is at, asshole?"

Thanks for the feedback. I won't leave the prepostions dangling :-)

Although as roy said it sounds strange.