I'm with you. I like my bags with good body. Here's what I do --
I use fusible fleece instead of batting. This, by itself, will give the main bag some body.
But to add a firmer feel (so it stands up by itself), I use a medium fusible interfacing on the lining. Don't use heavyweight, it will be too hard to turn through the lining hole if your pattern uses the "birthing" method.
For the bottom, I use plastic canvas.
These tips have come from trial and error over many bags (sell them locally and online and no complaints).
Hope this helps!