The preheated oven turned off method cooks the whole liner at once, in 15-20 minutes, not counting preheating time. Not sure what's so difficult about this that people would go looking for other science projects to heat them...
Molding the liner to the boot as well as your foot is very important, IMO.
I've cooked liners at home many many times, it's really not hard, and liners can be re-molded at least half a dozen times. The first time I got moldable liners I did take them to a shop. Guy didn't know what he was doing, despite telling me he does them all the time, and used heaters akin to hair dryers to heat just the inside, inadequately. This prompted me to learn how to DIY at home.
DIY.