That post was over 3 years ago and you may not get a reply from that poster. A saturated solution of Borax would be one level full measuring tablespoon of borax dissolved in a cup of water, but that may be too strong for people with sensitive skin. It may be better to first see how you tolerate one tablespoon dissolved in a quart or liter of water. You probably already know this, but borax water does not have the slimy feel of liquid bath soaps. It basically feels very similar to plain water so it may take some getting used to.
If you tolerate that mix and want to make it stronger, you can try a tablespoon of borax dissolved in a pint or 500 ml of water. You can use a 1 quart spray bottle as a dispenser, but this can feel cold to some people when spraying the skin. What I have found useful are bottles around the size of 16 ounces or 500 ml that have a flip top that when open has a small hole for dispensing the borax water into your free hand or wash cloth and rubbing that on the skin. Some 16 0unce bottles of rubbing alcohol have a flip top lid with a small hole. You can also use an old bath soap bottle that has a pump, but these tend to dispense to much borax water in one pump and so you waste more.
A little trial and error and you will quickly figure out the best dispenser for you. Use it as a regular shower soap when you shower. Keep an eye out for any negative skin reactions because borax has a fairly high pH at just over 9 while human skin has a significantly lower pH in the 5 to 6 range. Good luck and please update us on how it works for you.