funny you should mention Field's mod ... nudge, nudge.
it took alot of flight testing to perfect ... you might ask them to license the STC to you, rather than re-inventing the wheel. i'm sorry, but what i know about this is commercial confidential. i believe that they offer their observation window (replaces the type III exit door) for sale. they also have a gross weight increase STC (for S200), may be usefull.
good luck