Like the small sections of inner tubes we used to cut and slide down over the seatpost clamp on our steel race bikes, this BMC Streampost Seal will do an admirable job of keeping water and other contaminants from entering your bike frame from this vulnerable spot.
The Streampost Seal simply slides up onto the seatpost and pushes down over the opening into the frame once you've set the height of the post. It uses a wiper seal to provide the protection.
The BMC Streampost Seal is perhaps an easy part to lose under the workbench. Luckily they're inexpensive and made from a flexible, super durable rubber. Sold in singles.