That is how I made mine. I had some flexible rubber stuff that I cut and stapled to a piece of wood to cover the top. Mine is also about 3 feet high and holds almost a 50lb bag of food.
I'm fairly tall and don't have much problem pouring in the feed. Although, if I had a little better faith in my sawing ability, I'd probably put a wider top to ease filling.
To put additional stuff in, you just need to slide a 2x4 down the wall. Mine could easily be partitioned to accommodate starter/grower, oyster shells, laying mash, etc.