Thanks for the reply North.
Yes I realized the Century Wavemaster is going to slip around a lot by watching some videos of it being used. I'm not particularly strong but I'm confident my punches would most likely knock one of those on its side. Not to mention I'd like to do kicks as well.
Filling it with concrete would probably keep it from moving around but it'd be way too big of a hassle to transport it and not worth it for me. I was thinking of filling it was a mixture and sand and water. Mixing both, letting it sit for a night and fill it up some more until it's full. That way it'd be heavier than either on its own. In addition to that I was also going to duct tape the base to the floor so it wouldn't move around. Hopefully that would keep it stable enough for me to train on.
I've also been asking my roommate and some friends if they'd like to hold thai pads for me instead since a bag and a pair of pads are about the same price and pads are so much better for training. However, I'd like to practice at least once a day and no one around me can make that kind of commitment so I guess i'll have to settle on a bag for now.