I played around with Boa Constructor and WxWidgets back in the day, in fact it was the first cross platform GUI library I seriously looked at for Python, but in the end settled on PyQt. It's a lot more complete and capable, with excellent documentation.

I have always preferred to code my UIs though, rather than use a GUI designer. I just find the back-and-forth between graphical designer and code an annoying extra context switch every time. Also in code I can position elements exactly, or use sizers very fluidly where GUI designers are fiddly and imprecise. They can be useful to do an initial mockup, but not much more so than pencil and paper. YMMV, we all have different visual and mental cognitive models for these things.