When I have a complex design that I want to 3D print, I’ve found it convenient to create each detail as a separate solid group or component. Rather than try to combine all of them into some ultimate solid, I leave them un-exploded and export the groups or components to STL. As an example, I used arcs and the Follow Me tool to create a single solid:
I created multliple copies of this:
Adding a base plate:
This was done at 1000 to 1 scale, so I created copies of the components and scaled them down by 1/1000. Selecting the scaled down entities:
When imported into Slic3r, it handles the groups or components as separate shells (with no errors):
Not only does this make things a lot easier, but it also makes it much simpler to come back later and edit the component(s) and re-export everything as an STL file. There is no need to make the entire assembly a single solid each time.