I believe that simple straight out and straight down stretches, combined with a modest jelqing routine can increase base girth.
I attribute some of my base girth gains of about a half inch exactly to that.
Let me explain.
My routine has consisted of only manuals for nearly three years, specifically various stretches, jelqs, and squeezes. I’ve always warmed up for a couple of minutes with heat, done my stretching while flaccid, and then done my jelqing routine. I like to jelq at an erection level of about 30 to 40 per cent.
After I’m done jelqing, I do straight out and straight down stretches while still partially engorged, 25 to 30 per cent say. I’m not completely flaccid but still somewhat puffy from my jelqing.
When I start with the straight out stretching at this point in my routine I can actually see my base girth expanding as I’m doing the stretches. It’s pretty obvious to me what is happening. I don’t notice this type of expansion of my base girth when I do my flaccid stretching.
I believe this expansion as I’ve described has contributed at least somewhat to my base girth. And it’s consistent with what I’ve read here from some members who’ve gotten base girth gains from hanging and using an extender.
To the OP, this technique may be worth a try.