Filling Deep Channel Quickly

[Molebob]

I have a ~4cm deep channel with a plastic oval conduit in it. It runs about 1m up the wall. I normally would have mixed up some bonding plaster and done it but like an idiot I used some ready mix plaster I found at Wickes - after five days it hasn't set and so I just dug it all out (easily).

Now the question is, how do I get this channel filled tomorrow so I can be painting it the day after? Happy to do it in stages but not sure the best product to have it dried in time.

 

[StudWall]

When i re-did my bathroom and ripped out my old power shower i had a huge channel in the wall which i had to fill roughly to tile over, ended up using some ready mix rapid set concrete, screwed a piece of ply over the wall and poured it in the top. wasn't perfect but did a fine job enough to tile over the top