1. Go to a hobby store and purchase a sheet of colored plastic. Just make sure it is of a Mylar type. Cut to suite.
2. Go to almost any flea market or coin store. See if you can find a pile of cheap foreign coins and use them to fill the holes.
3.Go to a hardware store and purchase Copper Washers of a size that will fit. Being Copper they will stand out and not be confused with a coin. Some have Brass washers and they shine better too.
4. Not sure if they still carry those but a place called Harbor Frieght used to sell an assortment of Copper Washers. If so, order them and use for those fillers.
5. Or use Stainless Steel, Tinned or Zinc plated washers.
6. Fill the empty slots with a cotton ball
7. Pour concrete into the slots. Problem then would be how to get it out later.
8. Send incompleted Albums to me and stop worrying about those empty slots.