Thats what I was thinking. Thanks for your input.OK - now that you added a picture. My .02 Take off and throw away. Why would you want to limit how easily water can run out of the frame?
Worst thing you could possibly do. They should be left wide open.I'd take it to a welder if you want them plugged. I'd still leave the bottom an 1/2" or so open for fishing new wires and drainage.
If you still have any interest in a replacement part. They use them on winch posts. You can get one here if you like - http://www.easternmarine.com/Boat-Trailer-Winch-Posts-Mounts/ or just google plastic winch post cap and look for another seller.
You are correct...which is why they shouldn't be on there.No worse than rubber caps....