I change my brake pads out forracing pads before track events, then back to OEM pads for the street, so I have done this a number of times. As you noted, the rear piston screws in, doesn't just push in like the front pistons.
A 'brake cube' like they sell at the local autoparts store doesn't fit unless you grind off 2 of the nubs on the side that has 4 nubs, like this:
After that it fits OK but still doesn't turn easily and wants to pop out of the slots, making me worry that I was going to damage the piston or boot. So I decided to spend the $43 and get a proper tool, which makesthe jobmuch easier:
It's a decent tool for the price although of course the similar ones from Snap-On, Mac, etc are better but a good bit more expensive. But it fits well and makes me much less concerned about damaging the piston.