I'm trying to fit 4 25mm back boxes into walls. I've pepper potted the area with the drill then used a cold chisel to clear out the plaster and brick. However, the result is not smooth. There's a lumpy mess and some cracked brick that is stil somewhat firm, and the back box doesn't seat right. With one recess the back box is about 5mm too deep.

I'm wondering if I can build up the chase with some polyfilla or something to make it straight and the right depth before fitting the back box?

I know that an SDS drill with correct chiselling attachment would do a much neater job, but I don't have one to hand.

I've got 3 more holes to chisel out so any advice would be appreciated!

Cheers,
imanc