GroundPrimitive

I am looking to draw a volume as a ground primitive. I would like to have the flexability to define a volume where at least the top or bottom of the volume would be clamped to ground or a fixed height above the ground (AGL). If the bottom was clamped to ground or rendered at a fixed height above the terrain (AGL) I would like to be able to define the maxHeight to be a fixed/absolute value (~AMSL). Likewise if the top was clamp to ground or rendered at a fixed height above the terrain (AGL) I would like to be able to define the minHeight to be fixed/absolute value (~AMSL).

I was looking at the GroundPrimitive code and thought that I could pass in optional values to adjust the _minHeight and _maxHeight values around lines 1067 (near the terainExaggeration) in GroundPrimitive.js. However this did not seem to have any impact on what was rendered.

Is the GroundPrimitive the correct place to make this sort of change? Any ideas on how to accomplish what I would like to accomplish?

Thanks in advance

Jerry