Sponge would probably be ok, yea it will add nitrates but this is a FOwLR
. I don't think it would hurt too much.
I looked at all the pics and just saw the intake pump and PVC
for the skimmer so yea I understand why you can't put it there. If you wanted to add a baffle knowing this I'd go close to the pump intake and put a similar setup there as you have existing.
I do have one more little idea, Have you tried moving the pipes/tubes from the overflow closer to the wall oppposite the baffles? Meaning if you give it more space between where the water comes down from the tank and the baffles themselves then you might get rid of more of the bubbles. Not sure of what splash back you many have from that but a partical cover might fix that.