Considering all the Glock aftermarket stuff available (butt plugs, custom slide plates, custom mag bases, colored pins, ...), I am honestly shocked that no one has marketed a plug for the slide cut-out. If "Need" were the only reason to market Glock junk, 90% of the parts on the market wouldn't exist.
"You'll shoot your eye out." -Mom
"I want a laser for my gun." -my wife
G30, G20/21L, G34, G29, G27, G33, G23
Springfield Loaded (Black SS)
Sig P220 Stainless Elite