Fill closed spline

Is there a way to fill a closed spline with a colour? - Something similar to the curve mask node but using nulls as the points instead.
I have a four point extruded spline with the nulls of the spline attached to sound modifiers but I’d like there to be a 2D fill in the same shape as the closed spline behind it. What would be the best approach?

HI Did you have any luck with this? after the same thing…