Convert Polysurface To Work In GH

Hi All.

I'm pretty new to 3D modeling and even newer to Grasshopper. I have been using Grasshopper to create patterns which I bake and use for a boolean division in Rhino to decorate a boring object (for 3D printing). I have had some ideas though that would be a lot easier if I could put part of the objects into GH to manipulate as a surface, then bake it and use Join to make it solid again. I've tried various methods to convert the extracted surface from a polysurface to a single surface, but haven't had any luck. Any lofting I've tried will usually behave strangely or won't let me include all the surfaces. I also tried duplicating the part in GH but also having no success. I'm not sure if what I'm trying to make is possible, but if it is it seems like it should be pretty simple and I'm just not familiar enough with the program to do it.

Here's a link to the Rhino file. I'm just trying to make the top part, while keeping the outline shown in the Joint layer in tact so it can be re-joined. It would be nice to be able to adjust the height and fillet radius, but it's not really as necessary. I appreciate any help or suggestions.

https://www.dropbox.com/s/mafzfzg961jms7b/Blank%2058mm%20Scale%20Rh...