I have done this as well. The best way to get the cover back on is to lift the chunky rubber seal and push the plastic cover back in horizontally under the seal towards the window side then locate the plastic feet. If I remember correctly the feet fit in metal clips which must be located correctly into the dash side.
The reason these pieces are so difficult to reinstall is clearly that the dashes are pre fabricated and installed into the car. The thick rubber door trims are then installed over the dash end pieces.