DIY subwoofer cable, RCA, 30 feet long?

I'm building 2x 12" Rythmik subs and plan to play around with positioning. One of them is likely to end up far from the amp/pre and thus need a long IC. Connectors will need to be RCA. I live in a large city, downtown, so significant RFI can be expected. Actually, my REL sub makes a noice when my mobile phone sends/receives data or call...

The sub will get a line-level signal from my pre.

With this in mind, what should I look for in a bulk cable if I'm going to build a 30 ft IC with RCAs? OCC copper coax? Maybe I'm oversweating this and a regular/cheap coax would do just fine. Thinking coax for the shielding.

SVS subwoofers sell RCA interconnects in long lengths for very reasonable prices. Check their web site.
DH Labs DIY bulk cable (Sub-Sonic) made for subwoofers. I made a set & the copper braid is very thick. I combed it out & soldered on both sides of a Cardas RCA. It made a nice improvement.

Audioquest makes a sub cable w/a ground conductor you can attach to chassis at either or both ends. Audio Advisor has them for sale. Also check out HCM audio, as they are a big Audioquest seller.
I use a 30' subwoofer cable to my sub. Got it from BlueJeansCable dot com. Works great. No issues. Not expensive. No need to DIY.