How about this. Granted it's large but you can scale it down to whatever you need. Since I don't know what color your using for an alpha channel I can't help you much more than this. Basically the area around the shield needs to be your alpha channel color.
Use a color that won't be used in your skin. Make a text file with the skins main name followed immediately by _a.
Example: P51DSID_a . Save this text file and place it in your skins folder. Now I don't know what program your using for your skins but you need to get the index number. In Adobe Photoshop it's under the Info tab. It will say Idx and have a number next to it when you go over your skin. Save that number in the text file you created and everything that is your alpha color will become transparent