Safe geometry is usually the problem; that the sides, top, and bottom are larger than the door opening (generally).
If you have an existing shelf, and it clears the long arms, you could just use "floor mount" full-extension drawer guides fasented to the bottom with a self-made shelf; or a premade laminate shelf for that matter. (They are called "floor mount" as the bottom of a base cabinet is referred to as a "floor" in the casework trade; this is to contrast to side-mounted drawer guides.) Floor mount f/e guides are typically rated at 100 or 150# loading, so would be able to carry plenty of firearms stuff.
A note, examine the shelf in your safe, first--rathe more than a few are low-density flake board, with carpet stapled on. Those will barely hold a screww, let alone anything attached via a screw.
I've given some thought to tucking some narrow shelves along the side of my safe using the shelf standards already there. Then ginning up slide out rifle racks for the middle. Just sketches on a notepad at this point.