Use a form liner, everything goes on that. They use all kinds of stuff, thin ply, even this wacky plastic card-board, and textured stuff.
Or - I have only done this a few times when the situation was right. Brace/wedge VERY VERY firmly against re-bar, tie wire it to death! Works great on cielings. Doing it this way on walls is risky, but you need to make very sure that the rebar is tied tight to the snap-ties and pushes solid against the form, or its a disaster. If you dont know or doubt it in the slightest, use 1/4-20 rod to jamb it against the other side of the form. (But only is the other side is not a finish surface.)
Or - I think they call them "Bucks"? They are plastic tabs that you can put between the seams and attach the boxes to those. But the seams need to be where the boxes go....
Or - Fir the wall out!
Building felt can be used to keep slurry out. Jamb some in the box, then make a gasket, and cut tight to box....
For conduit I'll use anything but Smurf, never again....
