Flat copper is used because of the "skin effect". High frequency radio waves travel along the surface of a conductor. Therefore, the resistance (reactance) of a flat, large surface is far lower to radio waves.

Unfortunately, braid, tinned or not, will corrode much faster than solid copper.

BTW: It is a myth that the flat copper needs to be very thin. This myth is used by the people in the marine electronics supply business to sell very thin copper at very high prices. Thickness does not matter - surface area does. Very thin copper will have a shorter life in a marine environment than thicker stuff. Sooooo - the marine trades sell you less material, at higher prices, that you have to replace more often.