Quick, rough pictures: before, it looked like this. The sides with shelves just exposed.
I got two sets of black hinges at the store, and two pre-cut shelves meant to go in bookcases. I suppose I could have got plain wood and cut it exactly to fit, but this seemed easier. The shelves I turned into doors aren't long enough- there's a half-inch gap at the bottom, which I use to slide my finger under to open. And they're a bit crooked- I tried to measure and mark and hang exactly right. But the left door sticks a little against the center one, and the right one swings entirely free, I have to nudge it into place every time. Still, it's an improvement, and nobody looks close enough to notice the flaws (except maybe on the right, where the door is not straight with the main shelf and it looks like there is a gap under the tank).