I need some black knights for my VC army, but 2 problems with that, at least according to me

A) They're ugly
B ) They're expensive

So, what that in mind, I was thinking of converting some from my spare chaos knights. The idea was to swap heads with those from Grave Guards and use some of they weapons and shields as well, but the problem is, the knight body is so darn big compared to the heads and arms. Anyone tried this and gotten some good results? I was thinking of maybe using green stuff to fill in the gaps, but don't know how good it would look, seeing as I've never used green stuff before