I dont really have a problem with turning the cups either.
But should i choose silver or copper for the wire?
The cheaper one. Which means copper 99.9% of the time.
If you believe in cable material/sound difference theories, then copper is typically thought to warm and smoothen the sound opposed to silver that supposedly extend the highs and increase clarity (in general anyway, not always agreed up on), in which case copper would probably be better anyway since HD25 tend to be quite sharp in lower treble/treble area, and not something you likely want to boost.
Also, try to have the connecters to be of HD25 variety instead of the HD 600/650 ones. I find that the HD600/650 ones, although compatible, don't fit in as tightly and securely as the HD25 version due to size.
Nice mock-up heyheykuk :-)
This may well work, if as K_19 states, you can find a way to hold the right channel cable in place securely enough.
The suggestion to look into custom cable makers on this forum is also a good one, and probably you're best bet. If you can get someone to modify the stock HD 650 cable for you, you can try an upgrade to copper without too much out lay.
Yes, the original HD6x0 copper cable is well fit to HD25 with sound improvement by replacing the stock steel cable. This is a good start for most of HD25 users, the improvement is significant.