those HB C8019 have got a metal part, (molded into the plastic) that carry the output shaft. They use a other size bearing as well (which is good)16X8X5 instead of 16X10X5. the plastic is such a good quality, that you can make M3 thread in it, and crank the bolts really tight. (M3X18mm) they are the ones you want (about 5 dollars each) if you use a thin sheet of spring metal (such as .2mm thich) and make square plates from it, perforate it with your paper perforator, for the shafts to go thru, you will be able to build the toughest diff possible.
check
my diff, so you know what i am talking about with that square plates.