Mainly held by the two buckles on the front (looped through the rack) and the webbing strap on the back of the bag, passed around the rack bars. The rubber straps seem to keep it from wobbling, mine aren’t really taking any load. Has been fine when ridden then past few days
Not great pics sorry, but gives an idea. Couple of straps to fix it to bars, and then the main buckles looped through the bar