This company makes a plug for Glock grips that holds one round. Hey, never know when you might be one round short!
$8.99 and free shipping
Check his store for other calibers.
Mine is for a Glock 20 and it fits perfectly.
I am using it with gen 1 mags with plus 2 extensions. 1 in pipe, 16 in magazine, 2 in extension, and one in grip. Doing the math, my Glock 20, really is a 20! 20 rounds of 10mm is the way to go.