Make that a longer threaded barrel. The comp would have to be threaded to match the barrel and probably have to use a locknut to secure it, because it would have to be removed anytime you need to remove the barrel. T'would be ugly.

Frankly, it's a waste of money. Instead of a comp on a 9, do 20 pushups a day and about 20 concentration curls (with more than beverage cans).