diy tonneau cover question

I've got a custom canopy coming in October for my new long bed Tundra, but I want to take a road trip before that. I'm designing a lockable DIY tonneau cover. I think I can make it from a couple sheets of 4 x 8 x 3/4" plywood.

I plan to mount metal C channel to the bed rails and simply slide in the plywood in 3 sections, with the first and last sections having a hanging lower lip. So I slide them in to cover the bed, then lock the gate. The panels can't slide out, so my gear should be secure underneath.

My only question is how to attach the C channel to the bed. In the photos below you can see little rounded plastic rectangles in the top of the bed rails. They look like removable plugs. Under each one is what looks like hollow columns with a hole for a bolt.

They look very much like they were designed for a farmer to stick a 2 x 4 in and build a poor mans ladder or canoe rack.

**Can anyone confirm that these plastic caps are removable?




I'm thinking I can mount 2 x 4's in those holes, then mount my C channel to the tops of the 2 x 4's.

Anyone have any thoughts on this before I begin?

 

I forgot to mention...a little off topic...but I am really liking having my first full size truck!
My son and I loaded 6 of these 200# tree rounds in my long bed. The truck drove like it was empty. Sooo much power and stability. Not at all like my old Tacoma.

 

Yes those pockets are removable, but a 2x4 won’t fit in there. It’s meant for bull rings or a similar tie down https://www.bullringusa.com/

Also for the price of buying clamps and a few 2x4s and a few sheets of exterior 3/4” plywood and then the time to fabricate it, it might be easier to just purchase a lower end tonneau cover specifically made for the truck.

https://tonneaucoversworld.com/p/advantage-sure-fit-tonneau-cover/v/toyota/tundra/2017/

 

Can confirm the intention of those spaces within the bed rail. those are stake pockets, and intended to be filled with wood to box in the bed.

The caps on those pockets vary by truck. Some have removable plugs, some just an outline that can be cut out. On mine, there isn't even an outline! I suggest you check thoroughly before trying to get one out