Sealing milled cavities in masonry

Hello everyone :D

I have laid some electrical cables in my basement and for that I milled grooves into the masonry. The walls are made of cement hollow blocks (30 cm). By milling, I opened the cavities and now I am looking for a way to close them again. I thought about plastering but that is quite laborious because the cavities are so large. Does anyone have an idea how it could be easier?

Plastering over the slots would be the proper method. Anything else would be quite a makeshift solution that wouldn't necessarily look advantageous afterward; for example, I am thinking of chipping away the plaster around the slots and then closing the slots with mortared plasterboard of appropriate thickness (9mm?). You then fill these with putty, possibly over a mesh insert to the existing plaster.