We want 1 Iridium per 18 minutes and on average there is 1 Iridium per 5 chunks. Make a RP2 tunnel bore, 4 blocks wide and 64 blocks tall. We'll need to have it advance 1 block per 18*60/(4*16*5) = 3.375 seconds, which is easily doable. This "generator" will provide the equivalent of 5000 EU/t.