Are you talking about a mast?

You'll need to tap it. I would also consider putting a neoprene (the green ones) o-ring under the head of the screw so you won't feel the need to tighten it too much...that o-ring will also help to prevent the screw from backing out with trailer vibration.

I don't have any real data to back it up but the green o-rings seem to resist UV better than the black ones.

I also wouldn't directly tap anything that needed real holding strength into a composite structure...but you're fine in this case with a drain plug.