Pipe/tube for heater core bypass

[muhoo]

The heater core on my 98 E350 appears to be leaking, corroded, and plugged. I can't get to the thing without removing my dash and steering column and ignition. So, I just want to bypass it, at least until I feel like taking the dash off. Hardly ever gets cold here anyway.

What should I use? I can cut the heater hoses off at the heater core, but then I need to connect the two ends together. A straight pipe won't work, and even a 90 degree elbow might not. Do I need a U?

Also, what metal should it be: copper, black steel, stainless, aluminum, etc, in order to not mess up the chemistry or cause electrolysis?

Thanks.