Load STL then scribble on it with sculpt tool, deform/smooth a few places,

Load STL then scribble on it with sculpt tool, deform/smooth a few places, then export and print. Not bad.

Which software do you use to scribble ?

downloaded Cubify Sculpt trial http://cubify.com/sculpt

Check this out (free) :
http://stephaneginier.com/sculptgl/

Nice!

Meshmixer and Blender are free software that would work well for this too

Thanks, yes, I use both of them regularly and wouldn’t be without them.