The conical headgear of the KKK has its antecedents in that of the Straw Boys of Ireland (remember that many in the American South can trace Irish and Scots roots).

Similar costumes were worn by wren boys in Kerry.
There are also references to the White Boys and others.

The conical headgear (stiffened, perhaps, with leather or buckram) would diguise the wearer's height and the robes his clothing. Even the horses were sometimes robed, since they could also be recognized.

As for the fiery cross, I thought that it was carried by a runner in Scotland, not erected and set alight as the KKK does things.