is your tank full already?if its empty,just buy a peice of black plastic at homedepo or lowes and cut the teeth with a saw and silicone it in place.you should wait for the plumbing to be in place so you know you've made the acrylic big enough.better yet,they sell them already made.i think its called an overflow kit and their cheap.
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55 gallon reef with 4x54 watt t5,29 gallon sump,red monti cap,1blue echinophyllia,trumpet,red lobo,2 toadstools 1purple and 1neon,gsp,xenia and yellow xenia,red ,many mushrooms,2 giant frilly shrooms,cabbage coral,yellow polyps,many zoos,1 orange and one neon green nepthia,duncans,cladiella | 
