"Hi" Alternative, Yes, you are quite right, if you go down to the lumber yard it would not be inexpensive, but she would have friends who whould help her to find secondhand lumber (nice word that "lumber")instead buying new. Plywood could be renamed chipboard, 6"x2" lumber could be 4"x2" secondhand Fibreglass, well I dont think polystrine would help with the sound problem as you know, but its cheap so it could be used.
The hole in the ground where I live we have hot summers and cold winters, so forgive the effort of keeping warm in winter is uppermost in my mind. Thanks for the insight. John 2000.