How do I make a MTB shifter fit on a Road dropbar?
I'd recommend trying out some bar end shifters. You can get indexed ones for a few different gearing combinations. There are some ultegra bar end shifters on amazon for around $75usd. However, you could probably score some used ones cheaper via ebay. These are like what come on the surly cross check in the stock configuration.

You could mount regular rapidfire shifters on the top center part of the drop bars.  It might take some widening of the clamp to get it to work.  There is a thread on MTBR about an experience like this.