How to stack Face of the Mountain in practice mode.

After came out I picked him up and he pretty quickly became my favorite champion. I quickly realized that and passive would be a fun thing to play around in so I've started building it when queuing with my friend as my ADC. But as a barely silver level support I can't CS for my life and thus I've been using the practice tool to get better at CS'ing with 'ranged' attacks before noticing a problem.

I cant figure out how to stack in the practice mode. Doing some rudimentary math the game would need to just run for 54 minutes for the item to stack on it's own only giving me 6 minutes to mess around and practice. Am I missing something or is there no way to stack in the practice tool?