Ok, I'll bite since nobody else has, and maybe this will get things started. If you only want something to store hay and not equipment, you wouldn't need something very large just for storing 20-30 round bales. If you don't want to overbuild and know that's the only capacity you'll need, I'd suggest considering an all steel carport structure.
I built a 22x26 with 10’ sidewalls to pull two hay wagons through and get them out of the elements until they were unloaded. At the time it cost around $3k installed and took them 2.5 hr to set up. It has gables and looks like a regular pole barn from the outside except you get more clearance inside since there are no trusses. Later I went and had gable ends installed above the door way instead of a completely open gable. It let in too much wind and rain. .
You could actually fit that number of 4x5 rounds in this stacked on end, but you might want to go a little longer. Mine is drive thru but you would want three sided.