I am talking about the hole on the frame near the bottom bracket where the seat post exists (a Dahon folder) when the bike is folded.The reason is when riding in rain a lot of dirts get into the hole probably from splashing and when I fold the bike the seat post exists the hole and hence gets all the dirts in that hole. And when I unfold the bike and pull the seat post back again, the dirts on the seat post will be brought further up the seat tube and I can feel the particles in between the seat post and the tube, which damages the both the seat post and the tube.
So what I am thinking is to fashion some kind of plug so that when riding (in rain) I plug the hole so nothing gets into the hole and when folding I just unplug it so that seat post can pass. Anyone has similar experience or recommendation?