I won't give you a money back guarantee on this, but I've been able to turn the pistons quite successfully using a 4" c-clamp that can fit around the caliper. Extend the piston out using pedal pressure, but not so far that the piston pops out past the seal. Temporarily put in an old brake pad backing or something of similar thickness to act as a stop. Apart from giving you a surface to grip, pushing the piston out also puts a little brake fluid on the seal which will help lubricate the turning. Unbolt and swing the caliper out as needed for access. Support the caliper on a stool or something to prevent the brake lines from being strained while you work. Remove the piston dust cover by prying carefully around the base (or by removing the circlip on older calipers). Use a good quality clamp where the jaws won't twist out of alignment when tightened. The clamp faces must be smooth so as not to cause marring. Set the clamp on either side of the caliper piston. Make sure the grip is flat and centered on each side with the floating seat at a very slightly offset so it will tighten a bit more as you turn rather than slipping off. That last part is the trick to doing this properly. Turn the piston slowly and reset the clamp as needed. A piece of cardboard cut at the appropriate angle makes a good template for the piston face step. BTW Wrapping the clamp or piston in tape for protection isn't a good idea as the clamp jaw may more easily rip through the tape and scuff the piston. If you can find padding that won't slip or rip then okay, but metal-to-metal contact like the Volvo special tool is fine if you're careful.
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day