Tell that to the OP that is currently having the problem where there is an issue with the screws fitting
Hum, reading comprehension fail. If he had drilled out a nice clean 150mm core using the method I suggested then it would not have been a problem. However he chiseled it out using an SDS drill broke bricks etc. and now has insufficient space to drill holes for fixing.
To the OP fill the space between the ducting and the rough chiseled edge with a concrete mix made using 6mm gravel, leave for at least a week and then drill fixing holes for the vent.
