That first rack you mentioned is just fine for adult males and subadult females. I've also known people to keep their adult females in tubs that size their entire lives, but to each their own.
I wouldn't go with the larger cb70 until they're over 1500 grams because it's really just a waste of tub space for small snakes, IME.
The bonus on the sweaterbox rack is that Rich sells dividers which divide each tub into two, and is handy for keeping two smaller snakes in each tub until they're big enough for their own.
The flexwatt is recessed into the shelf, and there are two small blocks that raise the back of the tub up and clear from the flexwatt.
I've got 3 of that particular rack, and like it very much.