I wrote an addon to help extrude shapes on uneven surface. It is first release and there are a few improvements to be done.
Last release: https://github.com/hophead-ninja/perfect_shape/releases/latest
After testing and corrections addon will appear in blendermarket and own store - but payment will be optionally as a contribution. There will always be free and available on github.
Blender version: 2.76+
Ps. Sorry for my English