i'd strongly recommend any other course of action than repairing with bondo. especially so if you don't want the rust and damage to spread further and manifest itself in new and interesting ways.
wire wheel all that mess back and see what you're really dealing with. clean and treat the more solid parts with a inhibitor and encapsulator. cut out and stitch in new metal patches as needed. and, fair warning here, it may wind up being a significant portion of the foot well.
i know you said that you don't have a welder or know how to weld. well, this is the perfect opportunity! or, a great time to make friends or join a club and see if you can pull from their resource pool.
however, beyond the repair, it is critical you determine the source of the leak and repair that. otherwise you'll be right back in the same boat again.
good luck!