Gutter Mounts

[brushogger]

http://www.marksoffroad.net/1specials/rackclamp.html

$27ish/each

Those look like a copy of the original Con-Ferr mounts. Those were stout and rock solid. Glad to see someone is still making them. These are a bit pricey, but look like a good option. The bolt points down, so there is no nut or threads sticking up.
http://www.diamondrax.com/diamondrax-bulldog-rain-gutter-clamp