The materials to make HO scale "junk" vehicles are inexpensive:  I use metal vehicles as patterns, heavy duty aluminum foil, canned spray paint for an undercoating and latex paints from the craft store for finishing.
In 1998 I prepared this clinic as an article for a model railroad magazine who's editor showed interest.  I had received permission to use the ALLOY FORMS name and products from Mr. Jon Stetz of JAKs Industries. The article didn't get published then, so it's getting published here belatedly. I'd like to thank Jon for his permission and also his patience. I prefer Alloy Forms vehicles for two reasons:  
1) The detail on the cast metal models is incredibly fine. 
2) The cast metal holds up extremely well throughout the copying process shown in this clinic.