Convert Hollow STL to Solid

I already have a complete STL file, but it was originally exported as a hollow shell. For my upcoming 3-D print I need the entire volume closed so the mesh is fully solid—no internal cavities, single watertight manifold, and identical exterior dimensions to the original.

I’m not tied to any particular software; if you prefer Blender, Meshmixer, SolidWorks, or another repair toolchain, that’s fine as long as the end result is a clean, printable solid. I don’t need extra supports, wall-thickness tweaks, or other edits—just a faithful solid conversion of the existing geometry.

Deliverable:
• One STL file ready to send straight to a slicer for 3-D printing, verified as manifold/solid.

Please confirm how you plan to approach the repair and roughly how quickly you can turn it around once I send the file.