I haven't seen your other posts, but the 30x18 would be good for a sub- to small adult. If your rescue is a really small snake, then even the 24x12x12 would be pretty big. Recommended length at least equal to 1/2 snake's length, but more floor space is better for a larger snake, even tho they (balls) spend most of their time in hides.

My adults are in 42qt. sweater boxes or 30g breeder. My sub-adult is in a 20g long breeder, just fine. Just be sure to keep the lid covered to contain heat and humidity. Tanks take more maintenance work to keep them the right temp and humidity, but it IS doable. 5 of mine are in tanks.