A couple of ideas.
1) Get bulkheads and a small piece of appropriately sized PVC
and cap them off. (You might be able to just use a screw in cap as opposed to the PVC
and cap as well. I'm no plumber, but I know the stuff is out there to do it.)
2) Check out Glass-Holes.com dope aquarium stuff
which sells overflow kits that work through the sides or back of an aquarium. They also have return kits, but I'm not sure if they would be suitable for the hole you already have. Since you already have the holes, you want to make sure to order your kits without the hole saw to save some cash. I was looking at these the other day, and they seemed a little pricey to me, but I got a lot of chuckles from the guys who run this site. Their sense of humor is great. There might be some other businesses that offer a similar product, but this was the first one I came across.
(By the way, I ended up having to do a HOB
overflow since the craigslist tnak I got was tempered.)