I could give lessons in how to get the most out of safe storage. Let's just say I have WAY more guns in one safe than most ever imagine possible.
But those racks are nice!
It was mentioned about adding leg extensions. I offer this instead;
The shelves are adjustable in height. Get more shelf brackets, and build another shelf. Raise & lower the current shelves to where you gain the unused space. Then add another shelf.
Also, for the "poor boy" version of those plastic coated handgun holders. There is a kitchen accessory called a pot lid holder. They come in various sizes & a few of them are excellent for holding handguns. Wally world used to sell one that was perfect, and only cost about $4. I now scrounge the yard sales, thrift stores etc for them.