I would have thought that any kind of oil tank would be fine, even a nice shiny anodised ally one, just as long as you can carry the same quantity of oil as the standard tank. The only issue you may have if you relocate it somewhere too far away from its standard position may be that of increased or decreased hose length. I don't know if these changes would affect viscosity of oil needed or put any undue expectations on the oil pump. I have seen on a lot of V-Twin Chops where they do away with the Oil Tank all together, and increase the hose length to the car type oil filter which is mounted by the back axle. The larger oil filter and the increased length of oil hose takes the place of the Oil Tank.
Keep us posted of your progress, I have a '85 XT600 Ténéré thats getting close to needing a new oil tank, the standard one is nearly at the end of its life span. Drain plug is corroded in, every oil change I have to syphon all the oil out, nice quick job !!!