I want to put a drawer in it, so I bought a cutlery organizer made out of bamboo. It's just the tiniest bit wider than 17.5 inches, so I decided to ask the lumberyard to cut the top and bottom 20-1/8 inches instead of 20. (I'm hoping they're willing to do that precise a cut--as long as they're both the same, it won't matter much if they're 1/16" off or so.)

What am I forgetting? Have I done the math wrong?

Sorry it was confusing.

I finished the cabinet--took an entire day, with lots of fiddling with unfamiliar equipment, and translating what I wanted to my father-in-law. He was helping me, since I don't have a real workspace or much beyond hand power tools.

But if I do this again, I'm not going to cut the wood at his place; his table saw is too small, and he insists on cutting to a line, instead of setting the fence to the measurement. And we used his router in a way he hasn't used it before, so we were making up jigs as we went along.

But here it is!

It looks good painted.
Great for a first try and I know you learned a lot doing this! You should not give up now that you have some experience under your belt! Try a bank of small drawers!
Drawer boxes are something worth learning to do.

Congrats on your first project like this, was not easy.