Since you were just asking about a QT rack, I'd recommend building one that holds 15qt and 32qt tubs. You can add a hide for smaller hatchling in the 15qt, and a 32qt is sufficient for even a large female until it is time to move her into "general population". Double-check the tub measurements these days, but they should be the same height, so you can build it where it either holds 2-15s or 1-32 (lengthwise) per level. This way, you can switch tubs out based on your QT needs at the time.