Suggestions for hard lines for air lift trunk setup

As the subject implies...

Totally new to this area and am getting ready to install my Airlift 3P system. I am wanting to do hardlines in the trunk just for a little show and then transfer to the included plastic lines included with the kit using a union where it passes thru the trunk floor.

Any suggestions on copper vs. aluminum, vs stainless and fitting types to use would be appreciated.

Thanks