4.3 mm holes, 20 mm deep

[Chaz]

I need to do a set of 4.3 and 5.3 mm holes, at least 20mm deep.

I can drill these but will need to buy specific drill bits which may never get used again (around £15 each, carbide).

I could interpolate, problem is that a 3 or 4mm endmill typically only has a flute length of around 10mm max. There would likely be long reach endmills, will have to source.

Accuracy needs to be fairly good, how would you do this?