I do 50-100 at a time, and I think I have the perfect solution for you. Find someone (usually along a country road,) who sells big plastic barrels. The used 55 gallon blue ones that hold apple juice or other liquid foods are the most common, but my guy sells several different sizes. I got one that was used for pickles. It's about a 35 gallon container with an open top and a screw on lid. The screw on lid is the important part. I cleaned it out really well, fill it with water, then throw frozen milk jugs and bagged ice into it. It will easily hold 50 birds, water, ice, and salt. The ice lasts for days, and it cost me $7. Just buy 2 or 3 and you'll never need to spend hundreds on coolers of similar size.