When you drill the holes, seal them well. Buy a whole bunch of spare ones because they do break, and after a few years you probably won't find the same exact size any more. That's my thought.
first trip you go on, the plastic buckles will break and your cover will be flapping in the wind.
every cover that Dad ever had with the plastic snaps similar to those shown broke and he ended up with rubber bungee chords to hold the cover to the trailer.