So I got an asset from the Unity Asset store but it is useless for light baking because it is transparent on the exterior sides as seen in the attached picture.
You can’t do so without writing a special shader or doubling the number of tris. The reason for this is surface normals which you can look up separately (there is plenty of information about it).
Check this out for some “double-sided” shaders
Update:
Set shadows to cast 2-sided on your mesh renderer