Nothing to fix on the model I linked I assume, it was on the model you had, there were transparent polygons and such. New model looks fine I think.
I don't know how to go from sketchup to model converter, someone may chime in. All I know is you have to get rid of "alpha channels" in model converter.
Apply texture on both sides, there is a plugin: https://sketchucation.com/forums/viewtopic.php?f=323&t=30107
Of course this is for a sketchup a stand alone software, if you're using the web version I don't know how that works.
You can download standalone from here: https://help.sketchup.com/en/downloading-older-versions
Sketchup make 2017, that should work (I don't know because I use older versions)