Are you running the cables in conduit, how deep is the gap you are filling?
I covered my wiring conduit with about 1cm of sand/cement morter mix (With some PVA added) and then filled in the last couple of mm with ordinary builders plaster. It's not completely invisible but pretty good IMHO.

To finish I found you let the plaster go off a bit then spray some water on it. Using some left over mix and more water you create a thin slurry that you spread over the gap. Keep moving it around with the trowel and it fills in all the little holes and gives a really smooth finish, just make more slurry as it gets used up.
On a couple of runs I tried using one of those 'Easy DIY plasters' and it just went off too quickly for me to get a good finish.
HTH