Instead of the cost of a servo and having to hack it, these are available all over ebay...
http://www.ebay.com/itm/361063342336
Only $4 apiece ($16 for 4, free shipping, no tax). From China in 7 days, tracking info was provided, Paypal payment used....no problem.
Very small - see pic with AA battery - very smooth, quiet, very powerful. Used one already for a simple elevation unit.
Be sure to get the 6RPM at 3V model for elevation, turret rotation. They are available in many RPM ranges.
I'm amazed at how small and powerful they are...and cheap too.