Depending on size and scale these could look really good on top of sergeants or characters helmets. They get across the knightly idea whilst being subtly different from the roman and greek crests used by some other chapters. They come from the empire knights set.
Mk III helmets
These definitely look the part and would be perfect for as many marines as possible. I think the two on the right are from black templars and the one on the left is the company champion helmet from the command squad pack. So any ideas on how to get hold of them or model them cheaply?
A company called maxmini does a range of head swaps designed for "steam knights", I don't know who the steam knights are but they could be a perfect fit for some astral knights. The best options I could find are below