Thanks for the photo helping to illustrate you situation.
From my understanding, air does not need a significant taper in the front for it to be aerodynamic. A gap, as long as the surfaces are in alignment, I don't believe is significant at all either, the air simply skips over this, any airflow into or out of this area will not create an efficiency gain or loss which you would see at the pump.
I could be wrong, but I'd think any amount of money you would spend here would never be paid back to you in efficiency gains.
Again, this is just me.
I found this web site which seems to have a bunch of good info. Poke around, that list on the left seems to have a ton of info at each listing.
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