So I published this over on my site a while ago and didn't get round to making the instructable at the time, so it happens ages later because it's me. The distracted and confused guy.
On the upside I've had time to use them for a while and say that these are one of my favourite projects so far, they really look the part and they're really comfortable, not to mention they sound good, based on sennheiser CX(insert random numbers) they worked out well, the sound was a little fuller after the change, possibly due to the casing having more space and being a big solid thing for earbuds.
The design's uncomplicated, they're pot covers from maplins, thanks to the employee that actually knows where stock is, not to mention plays bass and had built an amp, so that was lucky.
A little black sugru was used to make them, rather than run the cable through the retainer screw hole I cut a groove and while not pictured I eventually put a little white sugru in the hole and labelled the left and right I made them with. (they need an angle to sit nicely) Incidentally getting sugru stuck to your ear is a possible side effect of the test fitting.