Peugeot 206 Trunk Dusting

Hello everyone. I bought a 2006 model 206 about four months ago. The vehicle has been damaged in various places. I suspect it was rear-ended, and there are gaps in the panels on the inside of the rear fenders, exposing the trunk to significant outdoor exposure. A few bodyworkers in the industry recommended sealing these gaps with silicone or foam, but I'm not entirely satisfied. I'm looking for your advice.

There may be a room and there are ventilation vents at the rear of the vehicle, but they may not be installed.