sometimes the top catch doesn't close the 1st time .. So double check before heading out. yes additional bottom hooks are sold .

holding the plastic nut on the inside and unscrewing the torx head screw lets you take the hardware off to add more hooks ..

a coin in a pair of pliers will get things tighter than the Knob..

Older type 1 top hook-rail had a buckle molded in and a strap that secured the bottom..

with a small amount of of ingenuity you can add a strap and buckle to any of them ,
with nylon webbing and a Fastex plastic buckle..

the Torx screw is pointed, it can pierce the webbing..

I have Paired front panniers with webbing , to come off as one.

I havent needed the harsh conditions secure mount myself .. but the principle is the same..