I thought building the biofilter is an easy job but it turned out it is not! What was suppose to be an hour job has now turned into half a day job and I'm not even finished yet. There are two problems I'm now currently facing. One is leaking and the other one is the water in the biofilter is syphoned back into the fish tank whenever the water pump is turned off.
I managed to solve the leaking problem, for now. The problem is caused by the 'ribs' at the bottom of the bucket (container) I used for the filter. The ribs are there to provide extra support to the base. I'm sorry I forgot to snap a picture. I'll update that later. So I sanded it down using my vibration sander and it seems that the problem is gone, for now. If it still leaks I would have to seal it with silicon. I'm trying to avoid using silicon for now as it might be bad for the fish.