Attempting 1st Camper Shell Build

I recently purchased a 2018 Tundra SR5 Dbl Cab 4x4... I will be adding an ARE Shell as well as a bed liner before attempting a camper style build out... I do not have a bed rail system, so I am contemplating using the bolt holes intended for the bed rail system (see photo) to anchor 2x4 railings for the cabinet sides... the rails will not need to support the bed in the middle, just the storage areas on the sides... so, being a newbie, is this a good idea, bad idea? Does any one have any thoughts or advice that might be helpful? Anyone know what size bolts to use? Anyone else done this with success... or failure? Thanks in advance for your time & thoughts!!

 

Those holes are 6mm and I would and do use UniStrut for this and not wood.
Strut comes in 2 profiles and I think the smaller or 3/4" profile is easily enough for your needs.
The great thing about Strut is that there are fittings and hardware that attach your stuff by bolting to track mounted fasteners.
They stock and sell the basics at the big box hardware stores.

For most uses they are all alike just with different names by manufacturer but anyone in Hardware will know what you are talking about.

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https://www.google.com/search?q=usi.....69i57j0l5.3599j0j7&sourceid=chrome&ie=UTF-8

 

8020 is another option

 

Thanks EzDog & Stig, I appreciate the suggestions and the info on the size!