Never used extrusion, but thought it was assembled with angle brackets and T bolts ?. if you want to drill a hole through the section you will require a steel sleeve(preferably hardened) of the required size, ie tapping or clearance. rough out a piece of oak or similar hard wood say 100mm long and square section larger than the slots in the profile, drill a hole slightly smaller than the OD of your sleeve in the centre at whatever distance from the end suits your needs and press in the bush, thickness the wood to get the bush dead central and a good fit in your extrusion slot and your hole will be central to the slot.
Regards
Mike