Hugin: roughly estimated as as 16mm “Thoby” lens, remapped without other distortion parameters to rectilinear projection and cropped to 133°x116°.
GIMP: paired parallel/divergent style, and filled holes with constant (ground) or horizontal linear gradient (sky) using averaged samples from image. Sky could be improved with a boundary condition based fill. Ground hole could be filled with artful copypasta or some such.
Basic VR viewing instructions: [link]