Take a threaded rod or tube - put a nut nearly at the end. Pop it through the top plate or vase cap - add another nut or approproate finial and finger tighten.

Half fill a large wine bottle with lead scraps or sand and put the rod and shade into it!

Starting to look a little like a lamp but really this gives you access 'in the round' to the bottom edge of the shade for that freedom to build downwards!

You could even start from the vase cap!