I did the same mod to my pair of DT770 Pros. I got my DT880s and missed my detachable cable, so....
Lumberg 3.5mm jack
V-Moda Cable :)
This is a link to the DT770 Detachable cable mod :)
Actually there is a very VERY small slit where air can leak. I don't want to use the same foam to seal it as I used on my DT770s, so I have to go out to a hobby store and find something that will fill the gap and harden. I'm not too concerned with it at the moment though, they are semi-open headphones anyway, air leak isnt going to affect the sound as much as it would on a closed headphone. As soon as I get something to fill the gap I'll update you on the size of the rectangular hole. The gap is less than .5mm wide.
The rectangular hole for the stock cable is precisely 11/32 of an inch long and 3/16 wide.
click on the "DIY Manufaktur" link in my signature. It shows the whole process.