It depends if the floor is flat but sloping.
If so, you could get away with option 1. fitting the plinth to floor height and ignore the gap above or pack with something appropriate to your skill /ocd.
If the floor is noticeably lumpy (or a better option from above) scribing the bottom edge to suit would be best. Option 3
It depends if the floor is flat but sloping.
If so, you could get away with option 1. fitting the plinth to floor height and ignore the gap above or pack with something appropriate to your skill /ocd.
If the floor is noticeably lumpy (or a better option from above) scribing the bottom edge to suit would be best. Option 3