If you're still shopping, look online for trailer lights. $10 sounds a little pricey to me, I've found the round lights with the grommet for $5-7 in local parts stores. Granted that doesn't have your flange, but they're LED which generally seems to be at least 2-3x the price for the same size lamp.
I found backing plates for my 7,000 Dexter axles online, got 6 of them for the price of TWO in a box store 90 miles away. And that included shipping!