Keep in mind that a defocused engrave doesn’t polish, it makes a more uniformly frosted appearance. If you truly want polished (transparent) voids, I think vapor/liquid solvent is your best option. It won’t be easy.
Another possibility would be to make your own layered construction. Get thin white acrylic, cut it out and transfer it to a clear backing using glue or double-sided tape. (See recent post about this layered approach)