I´m fairly new to the double thumbing technique and far from being very good at it. But i found out that I had reasonably problems to apply it on the g string, since my thumb has no further string to rest and bounce back from. Since I mostly play Jazz Basses with Pickguards, I figured out that one more layer of pickguard should help my thumb finding something to rebound from and execute the motion more accurate. Until now I have only installed a roughly cut piece of pickguard with double faced tape between the neck and neck pickup. But the outcome is pretty pleasant so far. Seems it helps. I will give it a try and post pictures as soon as I set it up properly. I may have to sand it down a bit to still have enough space to get my index and middle finger under the strings for popping. And of course I´d like to come up with a shape that is at least not too weird looking... Actually this is the first thread I started on TB. I hope it may be helpful for some.