If your mod is not multi-era(which means your banners will not change during game), I would suggest to get ride the grand-master's arms. I know G.Turner made nice designs showing the small shields with arms, like this:
But thats not important, Hospitallers were from great noble families, yes, but forgotten their social advantages entering in the Order.
Fortunatly, the historical banners have not changed from 1306 to 1500 circa.
the small cross you used is correct, and as said Spyros, that should replace the large cross on black background.
You can use the arrow banner as well for archer/slingers