hudradar.js

var boxMaterial = new THREE.LineBasicMaterial({
  color: new THREE.Color('red')
});

// Create the lines for the "2D box"
var boxGeometry = new THREE.Geometry();
boxGeometry.vertices.push(new THREE.Vector3(-1,  0,  0));
boxGeometry.vertices.push(new THREE.Vector3( 0,  1,  0));
boxGeometry.vertices.push(new THREE.Vector3( 1,  0,  0));
boxGeometry.vertices.push(new THREE.Vector3( 0, -1,  0));
boxGeometry.vertices.push(new THREE.Vector3(-1,  0,  0));
var box = new THREE.Line(boxGeometry, boxMaterial);
box.position.z = -100;

camera.add(box);

// Position the box over an arbitrary point elsewhere in space
var ratio = window.innerWidth/window.innerHeight;
var verticalFov = this.game.camera.fov;
var horizontalFov = Math.toDegrees(2 * Math.atan(Math.tan(Math.toRadians(verticalFov)/2) * (ratio)));

// Project to NDC
var pos = targetMesh.position.project(this.game.camera);

// Sorta project back?
box.position.x = pos.x * ratio * verticalFov;
box.position.y = pos.y * 1/ratio * horizontalFov;