I like the shape you're using now, looks good.

I like those style of deck fitting. I used to do them but they're so much work and very hard to maintain unless you just ignore the varnish and dirt build up. They also make varnishing harder. And edge wear on wood is hard to repair and turns black.

I tried a version with stainless steel bars but that got into another whole layer of metal fabrication of hardware and I nearly killed myself with a cadmium brazing rod.

Other than that they're great!

I applied wide tape over the whole area and hot glued the template in place. My decks aren't as curved at the sides as yours but you could add wedges top and bottom to firm up more. Quick to place and quick to remove with a dead blow mallet.

I used a smaller template with a smaller trim router.

Your results may very.