I hear you about wanting a bellyband...but if we had ours to do over again (and one day we might), we'd go ahead and fiberglass the seam entirely inside and out, solid and heavy. We'd likely make sure to include the wood inside to fasten things to, but it'd be sealed from the outside. Never again a leak or a worry about it unless the trailer were in another wreck.
Then, if we felt like it, we could add a decorative stripe or so, or paint
the whole belly area a slightly different color...but I think we'd just leave the
metal/plastic/whatever bellyband OFF.
Best with this. My husband, a machinist, said metal really is your best choice in his very humble opinion. IF you must have a band at all.
(A week after a complete redo, he was out recaulking the belly band where it had leaked slightly at one of the screws. So he's a little prejudiced.)
Just my two cents, YMMV, and I do hear you about wanting the band ON.
Photo of the rig before replacing the band. It's not a BAD look...