Well darn, I was all set to drink more wine and try again.
Yes, the putty is messy and has taken a while to set. I keep the thing flat when not in use. Putting the target in the fridge also works. The freezer would be even better, I suspect. But even soft, at the consistency of peanut butter, the stuff will absolutely stop pellets from either my B9 or RWS 34 at 10 yards in less than 1 in depth. The main hassle is just that the stuff, before it cures is sticky. And even after the surface cures, when you drill a bunch of holes into it, it gets an oozing wound. Have to be careful not to get it on your hands, then on your gun. The better solution would have been to buy the right kind of putty to begin with. But this was a fairly cheap lesson as these sort of things go.