Then glue the light switch base into the cutout on the interior wall with silicon glue. I imagine that a glue gun would also work in this case, but you´d have to be carefull no to melt any of the plastic parts. Also i had to file down some of the edges to make for an easier fit, with the dremel that´s a sinch. Once that was in place i tested out the connection again before closing it up completely. If the test is successfull then go ahead and glue the interior and back walls down onto the base with more silicon glue. The switch should fit tightly between the two walls.