I reply to myself. I finally got it:
@SubscribeEvent
public static void onRenderLastEvent(RenderLevelLastEvent event) {
Minecraft mc = Minecraft.getInstance();
LocalPlayer player = mc.player;
Camera cam = mc.gameRenderer.getMainCamera();
LevelRenderer renderer = event.getLevelRenderer();
PoseStack pose = event.getPoseStack();
MultiBufferSource.BufferSource buffer = mc.renderBuffers().bufferSource();
VertexConsumer vc = buffer.getBuffer(RenderType.lines());
Vec3 vec = player.getEyePosition(event.getPartialTick());
double px = vec.x;
double py = vec.y;
double pz = vec.z;
Vec3 camvec = cam.getPosition();
double d0 = camvec.x;
double d1 = camvec.y;
double d2 = camvec.z;
pose.translate(px-d0, py-d1, pz-d2);
LevelRenderer.renderLineBox(pose, vc, -5, -5, -5, 5, 5, 5, 0.9F, 0.9F, 0.9F, 1.0F, 0.5F, 0.5F, 0.5F);
renderer.renderLevel(pose, event.getPartialTick(), event.getStartNanos(), true, cam, mc.gameRenderer,
new LightTexture(mc.gameRenderer, mc), event.getProjectionMatrix());
}
Just had to call renderLevel at the end of the event handler.