Use the MCPCB as a guide and drill a couple of holes in the bottom of the Berocca tube. These have to be really very accurate - the lens won't fit if it's more than probably half a millimetre out.
The bolts (screws) here are 2mm and as you can see, there's hardly any clearance, but just enough, between their heads and the LED contacts.
I had a couple of accidents trying to bolt through the aluminum where the holes behind were not perfectly aligned. That meant the screw heads were not perfectly flat on the MCPCB and cut into it causing a short. Very bad, but if this happens repair it with nail varnish.
See Rob's Homemade LEDs for more about this.