A hammer helps, but...

The epoxy drills easily, no problems there. However, assuming M5 cap screws through nominal 6mm holes in the rails, you have a bit of wiggle room if you get the holes accurately placed. One way to do this is to clamp the rail in place as accurately as possible, then "spot" through the holes with, say, a 5.9mm drill just enough to leave a small mark in the epoxy surface. Then swap to the tapping size drill and pick up the mark to centre the drill in the hole. This is an old toolmakers' trick. However, it's quite difficult to hold the drill vertical when doing this, and if the tapping hole is slightly off then you end up with the cap head slightly off as well once fitted, which makes tweaking rail position more limited. You can do it with care, but I chose to turn up a steel guide bush with a spigot to fit the counter-bore in the rail and a central hole to guide the tapping size drill. One hand to hold the guide in place and the other to do the drilling worked OK for me. Saves a lot of drill-swapping as well. I used a similar guide bush for the tap. It's worth getting a couple (you'll need a spare, just in case) of spiral-point taps which will go straight through without the wind in/back off motion you need with hand taps. Stick one in a cordless drill on slow speed, use a touch of tapping compound, and the tapping is easier than the drilling.