Fix rust hole?

Greetings fellow

GREAT SHOW thank you I listen all the time

I have a rusted out hole hole about 10" long in the passenger side rocker panel of a 93 Suburu Legacy, the rust damage extends back a few inches into the underbody of the car. The local body shop will tack and weld a panel over the area for $400. Body shop says none of the damage weakens the structure of the car. Knowing nothing about this, and being broke, my prudent mind is wondering if it is possible to fill it with some spray foam and/or bondo? All it has to do is pass inspection in MA. The car has a lot of new everything so I would like to save it. thanks in advance I appreciate all input!