Measure the hight of the bath from the floor.
Go to Wicks or similar and buy some sheets of polystyrene or similar. Stuff most of the space between the underneath of the bath and floor with sheets of polystyrene then fill the space between the bottom of the bath and the top of the polystyrene with expanding foam, this will expand to fit the shape of the bath and will offer firm overall support.
Perry525
March 2008