I've rendered lamb fat a couple of times and never have too much of a problem with smell. I haven't tried it in a crock pot, but here's my method:
cut fat into chunks (we use shavings from lamb ribs that we feed to the dogs)
freeze
put through a meat grinder with a fine plate
put into a pot, add enough water to be at the same level with the fat
heat to a boil, reduce to a simmer
allow to cool, run through some cheese cloth
put in fridge overnight
The next morning there will be a thick layer of solid fat on top and liquid on the bottom. You can discard the liquid. I like to melt it again and filter through more cheese cloth, since you don't want meaty bits in your soap, I would presume.
I've had it boil over a couple of times, so just be careful. The stink isn't so bad with lamb, but maybe it's worse with beef. Either way, you can freeze it and it'll last a good long time.
EDIT: I grind it because it's easier to render down small bits than large chunks. If you can't grind but can finely chop it (it's definitely easier when frozen) then that'll work, too.