Take the top shelf/barrel holder out and build a new one with the proper hole spacing. Round holes are easiest to cut. Carpet the shelf over the whole surface. Glue the carpet to the shelf, slit the carpet at the holes, push the slit carpet through with the proper size utensil (I use a six ounce cheese glass) after putting glue on the under side of the slit carpet. I did a Browning document safe that had no shelves and it took no time at all. It holds 26 guns if none are bolt action rifles.