A completely encircled thrust structure is required to contain the ashes.  It might even force the ashes to the back.  There's a strange flow  of air currents.

The curved incense sticks have to be put in sideways to catch the ashes.  Sticks which are too thin to stand up have to be doubled with a toothpick or wire.  

Light the incense by 1st fully assembling it, then using the manipulator to angle it outside.  Light it & use the manipulator to angle it inside before it drops ash everywhere.  Many a lion has accidentally left it outside after lighting it, leaving a pile of ashes outside the starship.