Could you possibly post a pic of it? doesn't overly matter just interested.
I just stress to try and seperate your technique drills with your physical exhaustion drills.
Say have 30min of slowly drilling manouvering around the object, pay attention to how close you are to the object, think to yourself: am i leaving to much space for this guy to escape? obviously its a bag but were pretending its a real dude here right?
Throw your shots/elbows and knees to the body but mainly focus on where you are, and how tight you can be to the object, just try and base yourself on the bag whilst managing stability.
The next 30min just go all out really, your strikes/manouvering go at a high rate of pace, ground and pound the bag, judo throw that bastard
Exercise: place the bag behind your head and perform a back squat instead of using the barbell with weights just use that... youtube backsquat if your unsure of technique...
Military press... again youtube if your unsure.