So, it sounds like you have to remove the 10" side firing woofer, to start the conversion process. So, do the original passives have to be removed from within the cabinet? No way to remove the base of the speaker, and remove the passives, and then screw the blanks in from underneath? It seems like that would give you more room, and the blanks wouldn't interfere with the active woofers, as such. Or, why not just close the front facing port, in the base, to seal the cabinet. That'd slightly change the volume of the cabinet. But, you'd have to retune for the new 10" passive anyway...!
Good luck.