Kudos to Sapperwhite, the application is not marine and in fact, magnetic screws would be a great plus. Turns out that I'll probably need a #12 because neither of those guys carries them metric and a #10 may be too thin to do it--I'll have to check very carefully.
Of course, even McM doesn't carry #12, they jump from #10 to 1/4x20. Choice of material? I'll be lucky to get "metal".
I like the way Fastenal quietly generates a list with single screws for a dime, listed line by line mixed in with "qty 35000, $1994.65" two thousand dollars worth of screws. Gotta be careful about that "ADD TO CART" button!