i did the one on my old tacoma & just got an aftermarket replacement panel (yes they sell them), painted it, & Fusor(ed) it on there. as far as the front wall of the bed, they unbolt on most toyotas, so i got one from a yard.
Do you have pictures of what amount he cut out ? I mean if he cut any of the upper or side rear window channel it makes it alot harder and time consuming because of the bodywork involved afterwords .
Now if he only out the bottom of window channel and back wall , then it is alot easier and worth your time . As for building a rear wall compared to getting an original rear wall , I would say the New smooth rear wall is alot easier in fabrication and bodywork .