We used the IMT MAX-HD phantom (http://integratedmedicaltechnologies.us/index.html) with great results. We provided a white paper on the testing and the results for testing on the Vision RT Align RT system at AAPM 2017. If you are interested in the white paper you can email me privately and I can send it to you (mike.tallhamer@gmail.com).
Another anthropomorphic phantom with non-opaque surfaces you might try could be the Rando phantom