You are talking about the so called belly band. It does not hold the two halves together. They are connected by
fiberglass on the inside, after they are aligned and clamped together with a number of spring clamps. Mine, like all Scamps that I have seen, has an aluminum extrusion trim, held in place by small (1/8 dia.) pop rivets. Anything will work, as long as it looks good and stays in place. The one who gutted it before you, the gut-tor, probably looked for something that would not require any tooling or fixtures to make the corner bends, so he used rubber. That is my guess.