Velcro. heavy duty.
I've been thinking about doing something similar.
Whenever I have bought Shell engine oil from Opie Oils, it has come with a free grey zip up pouch about 10 inches long,, which has two strips of velcro already sewn to the back. I keep in it: cable ties, tape, wire, pliers, adjustable wrench, a couple of screwdrivers, spare relay plus other oddments. I stuck two strips of velcro onto the top of the footwell under the bonnet and it has stayed there with no issues.Better than drilling holes and no chance of water ingress.
