There is no set way to do it, but here is one way you can do it.
Start with a box mesh for now. Set the location to be roughly at your players feet and have the box be tall enough that it still stick all the way into the floor when your character stands on the steepest surface commonly found in your game.
Then in your material simply do WorldPositionBehindTranslucency and component mask for R,G and then subtract ObjectPosition.
That will give you coordinates centered on the characters feet where the center is 0. Now all we have to do is scale it to fit and bias so that 0.5 is the center for texture coordinates.
So divide by object radius, this should give you coordinates roughly going -1 to 1. Also put a multiply or divide by scalar parameter here for hand adjustment. Then multiply by 0.5 and add 1 to get centered coordinates.
Then just hook up the result as the UVs of any round alpha texture. There are a bunch of such textures in the engine content directory with various falloffs under the engine volumetrics folder.