I did not see any construction pictures so I am going to add a few more details.
Assuming the safe has removable shelfs (like a jewelers safe) or otherwise bare interior;
I measured a few rifle butts and 3 x 5 inch seems to be the optimum size, start by measuring the existing interior footprint, like 24" x 30", I then allowed about an 1/8" on the sides/front/back so there was no too tight to fit issues. I then determined how much "safe" space between rifles to prevent nicks and dings, IIRC allowed about 2" I then determined how many 3 x 5 rectangles fit the foot print and drew them out on the plywood base. I then drilled each corner of each rectangle so I had a start hole for a jigsaw and cut out the rectangles.
The final product was a 24" by 30" piece of 1/2" (or 3/4" plywood) with about 18 rectangles. This went over a thin sheet of 1/4" plywood covered it felt, the other thing formed the bottom of the safe interior. There are other approaches but this is what I did. You could simply carpet the bottom but the recessed cutouts insure butts don't move much and stuff doesn't ding other rifles.
I changed my sides from my original design. I first built up from the bottom with pine boards (like 3/4" by 2") to form sides and for the top I took 3/4" by 3" and used a hole saw to cut 1/2 circles every 3" or so, this is for the barrel to rest in.
I decided to change the design and use Neodymium ring magnets to hold the uprights, since the side of the safe was a steel shell it held as good or better than the original design.
The ring magnets are recessed for a screw and if you screw them to the uprights you just build to fit the design and pop it into place. With Neodymium magnets, especially bigger ones, it is not going anywhere. Above the rifles you leave a shelf or two (factory shelf) for pistols, optics, etc
As far as the ball transfer casters, this is an option but it is made so you can have a removable interior. The problem with regular casters is they will use 3 or 4" of valuable height in your safe. If you drill corner holes (like 1") in your base, and use ball transfer casters, you will lose about 1/2"
LINKS;
A good place to buy ring magnets, https://totalelement.com/collections/ring-magnets
The type of caster I am referring to is like in the pictures;
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