This question is difficult to answer. If GW behave like "normal" waves, obstacles in the inner [Fresnel zone][1] are the main nuisance. The diameter of possible objects should be larger than $\lambda/10$. Even big stars are smaller. The possibility of strengthening GW with [zone plates][2] is illusory.

Are GW "normal" waves?


  [1]: https://en.wikipedia.org/wiki/Fresnel_zone
  [2]: https://en.wikipedia.org/wiki/Zone_plate