I am planning the same thing. Last year I rebuilt my harddoors on the cheap and looked at just how easy it would be to do. I'm planning on using a template made from a stock Hard door. Then I can take transfer the pattern and have a metal shop plasma cut the material. Then I'll reinforce the inside of the door with a crashbar and a small frame and arm rest. It should be very, very easy to do. The aluminum cost will vary if you want stock diamond tred or bright aluminum. A thicker material will also drive up the cost as weight is a variable in the cost.
Good Jeepin'
Larry
Good Jeepin'
Larry