I've seen others make Screwdriver extensions as well, not that they sell them, just seen people make posts about them. I don't think there's anything wrong with stacking batts if you know what you're doing. For an unregulated mod, there's really only one viable configuration and thats with two LiFePO4 cells and those are safer than regular cells. You would not want to stack a couple regular ICO cells. The voltage is too high for anything other than a high resistance atomizer and you'd probably hit the battery's over-current protection with a standard resistance atty, if you could take the heat. The other thing you can do with an extension is run an 18650 which more than doubles the run time. I just don't see any real reason to be against an extension.