If I'm reading your post correctly - you want to match the corner pins of a layer in 3D space with the corners of another 2D layer? The best way to do this is with layer space transform expressions - but this is a complex area.
Lets's call your layer: "solid2D". On your upper left corner property paste this expression:
S2D = thisComp.layer("solid2D"); //get the name of the layer.
S2DP = S2D.toComp([0,0,0]); // get the layer's upper left position in Comp space.
fromCompToSurface(S2DP) // project that Comp position onto the surface of the 3D layer.
You will need to adjust the layer name: "solid2d", to your own layer name.
That should give you a matching position for the upper left corner pin.
For the other corner pins replace the second line of the expression as follows:
for upper right:
S2DP = S2D.toComp([S2D.width,0,0]);
for lower left:
S2DP = S2D.toComp([0,S2D.height,0]);
lower right:
S2DP = S2D.toComp([S2D.width,S2D.height,0]);
I hope the above helps!
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