How to shell macadamias without a special tool?
I've used C-clamps before.  You set the macademia in it, then tighten it down 'til it cracks.

You can keep a gloved hand around the nut while turning the screw with the other hand to avoid the risk of flying shell bits.  (but you should be turning slow enough that it doesn't crack explosively)

It does take some time, but it's less than a minute per nut.  It goes faster when they're all similarly sized, so you're not adjusting the clamp signicantly to fit the next one.