Be very careful with them, they're very old brittle plastic.
Each cap has 4 little horizontal bars that grab onto the plastic rail. Zoom this pic you'll see them.
Maybe use a credit card or something else that won't scratch and pull toward yourself and it should pop.
One of my custom ones popped off and was bobbling around up there for a few months before I realized it, (it can't fall off because the tower goes right through the custom hole). The plastic edge got really chewed up just from banging on the rail over and over.
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