Well, the bass should have little to know support by your left hand when you are holding it. Sometimes, you have to, but generaly you don't want your left hand to be a crutch. You just have to get used to reaching up there.
Now on my fingered extension I almost never grab the "grab spot" behind the extension. Unless, I am olding a D or a Eb out for a long period of time. Sometimes for the Eb I'll actually have my thumb behind the base of the scroll so I can do it first finger. Fingering wise, I can usually grab 1D with 2 on the Eb. Or 1Eb with 2 on the E. It is a bit of a stretch but I can get 1C# with 2D. Just get used to holding it there, and shifting around. Sometimes for fast passages I will 1 it the whole way.
To get used to it without changing your endpin height, or sitting to standing or standing to sitting. I'd just do vomits from the low D to E and G and A. Just things like this and practice quality notes and control of the bass.