Or if your tape measure is too shiny, use the plastic strips used to secure parcels together.
Fishing weights are great.
Sods law says there wil be a cavity tray, a double lintel, cavity wall insulation etc.
Trunking lid aint gonna work.
And you do know you aint meant to run cables in cavities

(In the eye of building regs, the cable could allow damp to track across the wall. In the eyes of NIC, the cables could be damaged during installation).