Mine is not secured (Shhhh) as I'm always taking it in and out but yes you could do that with bolts.
I used plastic bins as they are cheap and light...plus I have 4 different bins prepacked depending on what Im doing... (cooking , shopping, beach ,odds and sods) so I just swap them out depending on the day. Very quick and easy.
I was thinking the two items circled was a bolt or something through the fender. Thanks for the info on the bins. Basically easy and simple and can swap around as needed. Just pull them out and get what I need. Simpler than drawers and more cost effective. I have a LJ so even at 6” high will be able to keep a few things in the Jeep, tire inflator, battery jumper etc. and have some “security“.