Be hard without it, i found it hard with the bloody tool! If you can, its best just to pay the few pounds for one. I swore like fuck using the tool, as the piston was solid.
Not sure what else you could use on it without damaging the piston.
Make sure that once you wind it all the way back, the notch is at 90 degrees to the rest of the caliper if it is placed flat on a surface. This will help it retain it's position when the brake pad is in. You can hold the brake pad to the caliper to see how it would sit if uncertain.