How to achieve macro eye shot?

Hi there, I shot this closeup of an eye as a place holder for an extreme closeup of an eye I need to get at some point in the future for my film. it was shot on a black magic 4K using the sigma 18 - 35mm lens, this was about as close as I could get to the eye while keeping it in focus. I need the shot to be a lot closer to just focusing on the eye with the eye lids surrounding the frame out of focus and was looking into getting extension tubes for my 18-35mm lens, does anyone have any suggestions for how long I’d need the tubes to be to achieve the extreme closeup? Or any good brands? Thank you, hope this all makes sense.