A 5mm drill with more than 70mm drilling depth should be obtainable. Coming back to the thread title, I borrowed a picture from Stoney
http://stoneycnc.co.uk/stepcraft-cnc-videos/
, which shows an SC with spindle and tool fitted.
(Stoney, I hope you don't mind that I've borrowed your picture.)
To drill a 70mm deep hole into your part on the SC, you'd need to make sure that the distance 0 to 1 is high enough to let you drive your drill through the part into the sacrificial plate, so that the drill tip dives in completely in order to have the full 5mm diameter throughout the hole; and an air gap between drill and part, in order to avoid scratches.
If 0 to 1 is high enough is also influenced by 1 to 2 (min 70mm + tip), 2 to 3 (depends on the spindle you've chosen).
Alternatively, you could mill the part's contour and the wave shape, and just drill two pilot holes with a short bit; and then take the part to a drill press, and finish the deep holes.