Silicone is the thing to use. I built a sump/refugium out of an old tank and I used plexiglass as baffles. I also cut 1/2" strips of plexiglass as supports for the baffles. Put the supports on the side of the baffle opposite the water flow. So if the water is flowing from left to right, you want to put the supports on the right side of the baffle. The supports have a larger area for the silicone to hold onto and gives the baffles some support rather than just the silicone holding them in place.