Thanks for the info. I followed up more with the custom cable maker and he said he doesn't think he can get the Sennheiser connectors so therefore he can't make the cable. I could offer to mail him a cheap cable that has those connectors and then disassemble them, but I'm not sure if he'd accept that. And even if he did, I am not sure if it's a good idea since: a) it would cost me lots of money to send that cable to him and I'm sure he'd also (reasonably) charge me even more to disassemble it and b) I'm not sure if he'd do a good job since he's apparently never made a cable with that connector before.
When I search the internet for cables, the results I find either have very few reviews, or a lot of bad reviews, or they are posts from many years ago and talking about cables that are no longer for sale. I found one guy on Etsy who is held in high regard for making these cables, but his shop says he doesn't ship to my country, and also I don't like how he puts a very obtrusive label of the name of his shop onto the connectors he uses to make his cable.
So as a result of all that, I still have no idea as to what is the best way how I can solve the problem of getting a good quality & good value-for-money balanced cable for my HD6XX.