Mike, there is one of those very trailers in my back paddock which belongs to someone else but I'm sure I could borrow it anytime. Especially if I restore it for him
Tim, I'm glad you are finding this as entertaining as I am in the restoration.
David, There was a small piece of the gasket left which told me exactly what they used. It appears to be 1/8 thick x 1-3/4" wide and strengthened with canvas in the rubber. Purely as a gasket and the pieces that I made would have I presume a hollow core rubber glued around the inside to seal off for water. I will do both of those things after painting.
I have made the other door and hinge which is a very solid construction. I had to cut a piece of steel out of some RHS I had as it was the correct thickness but then had to run it though my rollers as it had a curve due its tensility.