I remember that article. I was going to post it, but I remembered it being quite an involved process so I didn't know whether or not it would work for what Tranman was after.
The unfortunate thing about well-engineered headphones is that a lot of what makes them perform so well is inherent to their design. A change from open/semi-open to closed is a fundamental shift--for one thing, you now have to deal with the backwave from the driver in a closed design. I'm sure there are other considerations as well, but I'd have to be an engineer and privy to the particulars of the DT880 design in order to say more. The DT770 is probably the closest thing to a closed DT880 from a design standpoint, and I'm sure Beyer didn't just close up the cups and leave it at that.