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I've printed a nice box (solid infill) like so: Unfortunately, this one, and others like it I've printed before started to crack soon after cooldown: Any idea why this is?
Is this edge curling?
Something else?

(ABS; 220C extr.; 100C bed; no fan)

It's very common with ABS, as it cools, it shrinks, pulling up the lower layers.
Printing hotter will often reduce the effect, you can go as hot as 240 with the stock Hotend, I'd try 230 then go up in 5 degree increments. The extra temperature results in a stronger bond between layers.
The other solution is less infill, less plastic means less warping force.

Polygonhell wrote:The other solution is less infill, less plastic means less warping force.

One of the main reasons I bought the 3D printer is to print enclosures, so I'll be doing that a lot.

Can somebody recommend some good practices for designing enclosures/boxes?

So far I've been doing this:
- 3mm wall thickness to get sturdy boxes (could go less if that's better)
- 3 perimeters
- 5 solid layers top & bottom
- 0.2 infill (usually)

what nozzle size?

with .5 nozzle 3mm wall thickness and 3 perimeters, your walls with have 0 infill (because it will do 3 perimeters on the inside and out). I usually try to get 1mm thickness on all sides, so what ever layer height that gets you close to that, 2 perimeters and w/e infill you think you need.