Installing Casters on Garage Cabinet: Advice

I finally got access to my garage again and am anxious to use this COVID downtime to setup some storage. I have 3 Gladiator Large Ready-To-Assemble Gearboxes, (72"H x 36"W x 18"D), and need to put them on casters, (it's a really small garage).

Before determining what I'll use as a base or which casters to go with, I'm stumped just trying to remove the female mount the leveling feet use. I took some photos but these seem like they're flared above and below the bottom panel. As in I can't just hammer it out. I'm not familiar with common mount systems, but does anyone know how I can disassemble these? Any advice would be greatly appreciated!

From the top of the bottom panel:

From the underside of the bottom panel: