Here's how I would do it. Copy/Paste the DP file from a similar aircraft into the new one. Rename the DP with the exact same file prefix as its AIR file. You can edit the DP with DPED Version 1.10 to position the weapons & damage boxes correctly. http://www.simviation.com/cfs2utilities1.htm
Each time you edit the DP delete the corresponding CDP file.

For external weapons to display & function, patch  the MDL with the MDL Converter Tool. http://www.microsoft.com/games/combatfs2/articles_SDK_mdlconverter.asp