When you say open coils, are you referring to spaced, or just saying the size of the loop?
When I said open I just meant big 2-3mm ID coils. Honestly on the Erl it seems like spaced work as well as micros. I've been using a spaced coil with my 30ga wire so it covers a little more length, but micro coils on 26ga.
I need to pick up some 28ga wire, as that seems to be the sweet spot for my Provari on this. 26ga takes a bit too long to heat up, but 30ga makes for a tiny coil even at 2ohms.