I have no knowledge of the usability, reliability and durability of Meikon housings, but the cost is of course going to be a lot less than for the excellent Nauticam ones. I agree that the Sony 10-18 is a fairly affordable lens that delivers pretty good results together with a degree of flexibility, and it is what my daughter uses for much of her underwater wide-angle work when she isn't using a fisheye. In my opinion it would be a better option than the Z 16/70 for most wide angle work because of its much wider field of view and ability to focus on subjects close to the dome, but of course the OP already has the Z 16/70. Incidentally the Sony 10/18 also works well behind a 7" dome port on the Nauticam system. The 10/18 is a bit too wide for fish portraits apart from the very big or perhaps some bottom dwellers that allow you to get very close. It all comes down to what type of shots you are looking for.
Of course Ikelite also already produce a plastic interchangeable port/lens housing for the A6300 that is substantially cheaper than the Nauticam system: http://www.divephoto...kelite-housing/
When I changed from an Ikelite system to a Nauticam system a few years ago, the improvement in ergonomics and pleasure of use was quite considerable.