How deep is the 12 inch BlockDock cabinet? I want to try a different speaker.

The BlockDock speaker is front mounted, and the distance from front of baffle to inside rear panel is 5.98 inches. (15.2 cm)

If your speaker is taller than this, you will need to route the rear port panel out or make a cutout larger than your magnet circumference. 

(Note: there aren't very many speakers deeper than this. Mostly Alnico, EV12L, or Black Widow speakers.)