Hello, Vincent. If you look at the picture you'll see that I cut a section out of the O-ring to make it fit. Or do you mean diameter of the cross-section? Either way, I don't know where they came from - they were just lying around the garage.
Main interplane struts:
I don't know what methods other people use but this is how I have solved the problem of getting enough strength into them. From left to right:
Plastic strip - cut in two - cyano-glued to a length of .5mm brass rod - cyano glue liberally dripped on - hardended glue sanded - strut painted Humbrol 110 - strut overpainted with Tamiya clear orange and ends painted black. The brass wire will be trimmed to the exact length before fitting and, of course, the last four steps repeated for the other side of each strut.