I don't think you have fallen far from the mark and I like the idea of having heavenly elements on top and hellish elements on the bottom. I too am tackling the task of this style of map, however, mine is more fantasy themed. During my research of this style, i found that the artists of old would fill the four empty corners of the canvas with depictions of different places, or the four seasons, or the elements fire, water, air, and earth among other things.
What do you plan on filling in the gaps with? Such as the area between the cherubim and the edges of the hemispheres. I think some clouds or some flowing cloth would look good. Maybe some sort of ivy growing up to help symbolize life.
I'm looking forward to seeing more and I'm glad to see someone else taking on a large project like this. it keeps me motivated.