Safety deposit box
The phrase referring to a box in which valuables are stored is “safe-deposit box” because it’s a box in which you can make a safe deposit. Not a safety deposit. But this eggcorn is highly understandable because when you say “safe-deposit box” aloud, the first two syllables run together to sound exactly like “safety.”
We assume you’re using “supposably” to mean “according to what many believe.” If so, then the word you’re actually looking for is “supposedly.”
If what you mean is “without a shadow a doubt,” then you have two choices, and neither of them is “undoubtably.” You can say either “undoubtedly” or “indubitably.” Either one is correct. Just don’t mash them together to create an eggcorn.