Having drilled two blocks with a 6mm holes too, line them up with the sheet-metal brackets by using a bolt of the same size. Then remove the bolt carefully in readiness for drilling with a three-quarter inch bit. You can use G-clamps to secure prior, else simply stand on it as I did. It's the only way (without a laser-cutter or CNC machine in your garage) that you'll be able to use a larger drill-bit without it launching the hand-drill across the floor at speed.
