Whenever I attempt to add custom hulls I get the hull I want inside a black box. Any ideas?
Could be a problem with the dds file itself. No better said, it certainly has to be a problem with the dds. I would advise you to use the nVidia plugin for Photoshop to create your dds files, if you do not own this program… (who does anyway?) you should use paint.net to create a dds file out of a normal png file with no background. If you already use paint.net or whatever try to create a dds file with a different file format… they all have cryptic names like A8B8G8R8 or whatever… you have to try what works best… or better said which one delivers without black boxes 
sorry for offtopic but…
HOLY CRAP
Brutix and a Merlin…
i love you 
now on topic… i have almost no experience in this but… are you using the same background color for the sprite as the vanilla ships have?
i once tried creating some sprites and i accidentally used a color different by like 1 degree of shade,it looked almost identical so i didnt notice
prolly a stupid suggestion but meh 
Yeah no well this has nothing to with the background color. There is an alpha channel in the dds file itself. If there is no alpha channel included due to whatever reason there will be generated those black boxes. So the problem might be that there is no alpha channel in his dds files…
This definitely looks like a dds file saved with no alpha channel.