NB, your suggestion above is about what I'm thinking, but I have some reservations still. On completing your last request for a model, which did not change the height, I saw a change in the current flow on the mast...that I did not expect. So, I asked for more study time.
Your next idea, using an insulator, is another complex consideration as well. I can't hardly describe, in words, what I'm think sometimes, and now you're throwing another monkey wrench into the mix. 
BTW, I think 4"-6" inches will do just fine.
Based on some modeling I've done using isolation, and some models that I've already posted, I think the choke and isolation idea could help eliminate RF on the mast below, and that is a good thing. This idea also seems to make sense to me, but to be honest I've not experienced any telling results...in my limited and possibly incorrect application for this idea. The particular isolation application may mean everything to the sucess or lack of sucess.
I know that Bob85 and Multimode200 have done it, and I think they will say the results were rather remarkable.
My Eznec somehow uses a Mininec algorithm, that I have to select, in order to handle a mast attached to Earth. When I do a free space model, I have to delete the mast, set the model to free space, set wire loss to zero, and the pattern plot type to 3 dimension. If you don't remove all the losses from the model, free space either won't work, or it gives wrong results warning that the model has losses.
No losses are the magic that free space models operate on, and for me this is why it's the best model for comparing antennas.
If my understanding of a free space process is correct, there needs to be no connection to the Earth, no environmental effects, no Earth effects, no resistance loss, and matching issues are not a consideration. It is just metal vs. metal, and design vs. design.
If I get into isolating some of the models in this thread and or doing free space models, things including my production and posting will get complicated fast.
I still have not heard from 954's Elmer buddy that was going to get back with an important aspect of my modeling procedures. So I still don't have, what I consider, near enough accurate model to really depend on this I-10K model. I made a post showing, in hand written notes, all the questions I have in this model already. So allow me some more time and maybe he'll answer.
