Alright I’ve got something close enough to work, though there’s a bit of aberration at the top and bottom of the sphere where the texture comes together, but not very noticeable unless you’re looking for it. To anyone who stumbles across this thread here’s what I got to work:
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Download the flexify2 plugin for Photoshop from the link posted by LexLuthor1.
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Arrange the 6 faces of the cube into the “horizontal cross” arrangement as per the flexify manual.
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Apply flexify to that image with the input as “horizontal cross” and the output as “equirectangular” (this is the latlong/HDR format referred to by Ryan B) and save as a png.
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Import this png into Unreal as a texture.
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Get Blender if you don’t have it.
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In blender make a primitive sphere, then invert the normals and export as .fbx
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Import the fbx into Unreal as a static mesh.
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Make a material out of the equirectangular texture and reroute the output from the texture node into the “emissive” input instead of the “base color” input. Set the lighting on this material to “unlit”.
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Drag the inverted sphere into the scene, scale it until it’s enormous, and apply the texture.
Should be good enough to work with. Also it’ll give you an error about UV wrapping when you build lighting . Still works for me so at the moment I’m just ignoring the error.