Chris Bartram G4DGU
The problem (for me) with components involving metasurfaces, such as in a a gap waveguide is that they need a different sort of approach to conventional TEM/TM/TE waveguides. Although I'm more-or-less comfortable with conventional waveguide structures, I don't feel competent to give an answer, as my involvement with metasurfaces has been rather cursory.
There is quite a literature on the subject, and it is probably worth starting a search on-line away from the amateur literature. If you can find back issues of the IEEE Microwave Theory and Techniques Society Magazine, you may well be able to find an introductory paper. I think I remember reading rapidly through one a couple of years ago. There are also a several books on metasurfaces - but be prepared for some difficult (at least for me!) maths.
Iin another life, I did have some experience of electroforming. Trom what I remember, copper requires a lot of care, and the careful selection of plating currents to get to a suitably small 'grain' size in the deposited metal. Particularly am mmWave frequencies, this will be very important in getting the lowest losses in an electroformed structure.
I hope these are useful comments.
73 Chris G4DGU