A 6in. gap is a bit more than I've dealt with but...
Originally my boat had the 3in. gap covered by a run of 1/4in teak
mounted with screws. It lasted 20 years. But water
ingress warped and de-laminated the stuff. If I had coated it in two part epoxy
when I bought my boat it would still be there.
Teak veneer plywood
is not terribly expensive, but if you can live with 3 in. baseboard molding (that you can stain to suit your color decor), you'll save a lot of $.
I don't see a 'lip' to attach it to on your boat, so Velcro sounds like a good option.