i thought this drawer would be quick but i keep messing stuff up.
first, i lost the dimensions i calculated when i made the first drawer. so i just measured that drawer, but i got the width wrong by ~5mm, which is enough that it won't attach to the rails right. and i made it too small, of course, so i had to cut a new piece of wood.
after that, i obviously paid a lot more attention to measuring everything twice. so naturally, i got it wrong on the one constraint i didn't check, and made a parallelogram. this is the situation in the picture: the sides are parallel and fit the rails, but the front is not square to the sides.
i have now removed the bottom again, and put all four corners in corner clamps. i'm going to let it sit while i take a break for dinner, then meticulously check everything again, and attach a new bottom — so the existing holes won't pull stuff out of square again.
i guess the lesson here is that i really need to keep enforcing all the constraints. it's not enough to screw each joint together squarely; i should clamp the whole frame together, check it as a whole, and then assemble.