Do you have access to a welder? Mig or stick preferably. They make covers for the lower door panels, but if you cant find some big enough, you can buy sheets of thin steel and make your own.
These aren't too hard to do the outside, just bend a curve into it, for the inside you can use aluminum or something else, but it wont matter as no one will see it.
Thats the cheap way.
Another option is to goto a junkyard with a grinder, find some pretty clean doors and grind the parts you want off, that way you're paying for sheet metal, not the whole door.