How to change the inventory icon for a 3D model block?

I have a custom block made from a larger 3D model. It takes up 3 blocks in width. Unfortunately, the icon for it spans over into the adjacent inventory blocks (see attached image). Is there some way to make a custom icon for it to override the default one? Normally, it isn't a problem, since the blocks are all the same size. But in this case, I need to figure out a way to do it if possible. Thanks!

http://scottburkett.com/uploads/mcss.png