I like the Ego One mega tanks too. I have three of them and use the 0.5 ohm clr coil heads. They're much better than the stock coils and rebuildable.
For the gap I get with some tanks I just cut out a circle of craft foam and punch a hole for the 510 connector. It pads the mod from scratches, stops any leaks I may get from going into the mod and fills the gap nicely.
I did notice that the black Ego One Mega base didn't thread as smoothly as the stainless base that I have. It was the same with both black ones. They did thread but kind of made me feel as if they would cross thread if I wasn't very careful. The stainless wasn't like that at all.