They have really nice real wood veneers, pre-glued and plain, in rolls and sheets. The laminates for about $35 a 2x4 sheet, available on-site. I think if you go to customer service they have all the samples on a wall.
Also, a friend of mine tipped me off to Grainger, I guess it's where the pros shop for tools & equipment. It's in the same neighborhood, in fact I think it's on the same block as Youngblood.
Another tip: Across Central, a couple blocks away in the Northup King Building is my said friend's studio- Danish Teak Classics. If you go in there, you'll find a the best upholsterer in town. He was an architect, then became an upholsterer.
If you go to this neighborhood this weekend, be prepared to hunt for parking- it's the annual Art-a_Whirl Art Crawl. Super fun, but very busy. You will find trades people with hobbies needed to help you fix your trailer, like the welders. They are at the Waterbury Building.
Have you thought about putting a formica type product on the interior of the door? Not sure how you would hold it in place though. Probably contact cement and pop rivets? Anyway, Youngblood Lumber on Central Ave (by 13th I think) carries a multitude of different colored laminate. I forget the exact brand - but it's not Formica. Kay says it starts with a "N", which is more than I remembered...