A MOI newby challenge that I don't know how to fix : )



Having made it, I realised the highlighted tapered section should be lower in position - all other elements are OK.
Info palette confirms it's a solid to start with. I tried separating those faces, moving them down, selecting the open edges and making a straight loft to fill the gap (open), then rejoining everything. I end up with 'joined surfaces' :(

I want a solid.

What's the right way to edit this?