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?

Try almost any real hardware store as they have a variety of rubber stoppers and cork plugs. Shouldn't be hard to find. Don't go to a big box store. Roger

Duct tape.

Wine and beer making supply retailer will have several sizes of plugs.

Maybe a basin plug on a piece of thin shock cord, tied to the seatpost in a suitable way. That way when the seatpost is raised, the plug does its work, and when folding, it becomes part of the seatpost.

I like this idea. Also the duct tape can be emergency use. Thanks everyone.