I've actually taken everything from ambientCG and converted it.
For OpenSim PBR, I've run them through a converter, MakeGLTF. For Blinn-Phong, I've inverted the PBR roughness map in GIMP to get the specular map, and I've used the unmodified NormalGL map as the normal map.
But I could apply the PBR material to the box that contains both.
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