Covering them up is not a good idea. Better to hide them behind something. They need water flow from all sides to work properly.
I have 2 sponge filters in my 125 gallon tank that I really don't like looking at them. I bought 2 artificial rocks to cover them up. Now the question is, should I just open the top or open the top & drill holes on the rear so there's water flowing inside the rock? What would be best? Thanks in advance.
Covering them up is not a good idea. Better to hide them behind something. They need water flow from all sides to work properly.
Mama Bear
My experience: I have learned always to make maintenance easier. In the beginning I also did not like the look of sponge filters and tried to hide them behind driftwood and what not. It was a pain to have access to the sponges to clean them periodically. Now I just put a breeding cone in front them, as suggested by Zach. And as Liz says, their performance might be affected by limiting water access to them.
There are several videos in YouTube where explain how to hidde sponge filters with plants; the idea is cover the sponge with little branches.
I have not done, but maybe it can works. The problem is you should to remove periodically the branches.