Help with filling large gaps

I have pedestal model that I am working on but I am running into issue of joining or filling. If you look at the attached file I have two main flat platforms, I want to join a mesh around entire perimeters joining both the top and bottom flat surfaces. After that I can subdivide and better play with form. Issue is that I cannot get any type of fill or join to work. What is best way to do this? It is ok if interior gets messy because I am going to export to a STL for 3D printing, just need flat top and bottom areas.Base Try 1.4.blend (467 KB)

First step, clean up the dreadfull mess. The only place for rubbish is in a bin
X / limited dissolve so the top and bottom faces are each a single face
Delete all but the top and bottom faces
Select the two faces and W / Bridge Edge Loops
In toolshelf or F6 adjust settings to whatever you think fits best what you want the look to be

Now to can edit further with normal mesh editing tools

Thank you so much for these details, I knew there was an easier way than what I was trying to do.