I presume you mean the honeycomb tank protection above the tank? It took me ages the first time, but now takes seconds. I afraid it is about angle and position! I know that is no help to you but keep trying and it will go in. Put some masking tape on any exposed paint and take care not to dent the car boot skin from the inside.
Really difficult to describe but here goes. Insert it from the offside, lengthwise in a near vertical position in between the main part of the roll bar and its rear angled supports. In plan view, it is diagonally across the boot space from behind the damper cover on the off side and the rear corner on the near side. There is just enough room for it to go.
If you still can't get it in I can post a video clip...