which ones are (actually) functional, and look good.

i wont hate one those i dont like.

but i need a bit more force back there.

please post pics.

 

Functional to what point or purpose ? and looks good is not easily definable as it is subjective.

I would say most of the generic spoilers could be made to achieve whatever results you wish with some testing, tuning and modification.
Obviously some spoilers will be a better starting point for your specific goals and aesthetic taste

 

Maybe you should ask in the Race section. I'm sure they'd have more knowledge of the functional type of spoiler. Most look good anyways.