You don't need to so much fabricate a top. Since you are doing DWC, you can use styrofoam unless you just have a lid for your 5 gallon bucket. The other thing is that in order to make your buckets light tight. You can use metal tape. It's about $10 for a 50 yd by 1" roll. You can put foil over the top of the lid in order to make it accessible. As for air tight, your bucket is never going to be airtight. The mesh pot lets air out as the pump produces it and releases the gases. Yet, you do want to keep light out or you'll start growing an algae farm in your buckets. The other item about DWc is try to keep the water cool. If you can't keep the water cool enough, you may need to add hydrogen peroxide or an additive like hygrozyme or hydroguard.