The alternatives to silver, silver plate, or nickle, would be brass, gold plate, or gold. I don't think wood would ever look quite right, and because of its brittle nature, one would run the risk of having it split, or fracture. I've seen a few dress sporrans with leather cantles, but the only one that really seemed to pull it off was decorated with a cast silver coat of arms, complete with supporters, and some amazing silver tack decoration. The best option, in my opinion, if you are looking to avoid the "shiny metal" look is a critter with full a mask-- muskrat, otter, and badger being the traditional choices. Even at that, I'd be tempted to have a silver shield engraved with my arms, crest, or monogram tacked on the critter's head just to liven things up a bit.