Thus, creation of a case of any type will be within your powers if you have patience, and process will take place with good mood.
As for the last, this locker had not to store simply laundry detergent, etc., but also all bathing accessories which stood simply on a bathroom before, with convenient access and would do them almost visible!!!
Took plastic legs, self-tapping screws and konfirmata from old stocks.
For creation of a case in a bathroom floor the hands you need to understand that it will constantly be under the influence of change of temperature and amount of moisture. There are some examples of what it can consist.
This kind of furniture, perhaps, will demand bigger amount of efforts. You should make some boxes of any height of materials MDF or chipboard. Further to do the screw gun an opening in the center from below, except the lower box, and from above, except the top.