yesssss, finally got something right for once.

i don't know if IEM's need it more than headphones. i just know most IEM's use diffused-field still more often than full-sized headphones. i think most current full-size headphones are computer generated and very few still use the head dummy diffused-field approach. only last headphones i know went diffused-field route was sennheiser's audiophile/reference line-up with the hd600's and 650's. the hd800's use diffused field but sennheisers own method of it. IEM's i believe use the more traditional approach of diffuse-field eq'ing   

You might wanna have a look at some couplers, the iec711 or zwislocki would be the best, expensive though, but they are said to be the most capable of simulating real ears.
tube length is one thing, but how are you gonna recreate the effects of having flesh inside our ears?

ballistic gel?

tasty!

LOL, I'm pretty sure the IEC711 spec couplers don't simulate flesh either. But they do approximate acoustic impedance of the ear more accurately hence more accurate results.

1/4 inner diameter of differing lengths - with a bundle of silcone tape to keep the seal.