This really isn't that hard*- a good fabricator should be able to take care of you. Figure where you want it and ensure it will fit in that position i.e. rear clearance as well as something to tie support bracing into. Decide which way you want it to open- up or down is going to be easier. Mark it out, tape it off and use an air powered hacksaw with a thin blade to trim the piece out. Save this piece. Clean up all of the edges. Fit your storage box, secure your mounting, mount your cut piece to your hinge, put a lock or two on it and call it good. That's way over simplified, but it's not rocket science if you have the tools.
I think I would use a CNC machine to make a backing/surround for the piece I cut out so as to strengthen it though. I would use one of the new epoxies to bond this to the factory metal. If I had autocad it would be pretty easy to draw up but I'm limited on bandwidth over here (working offshore). I'll see if I can't sketch something up by hand and scan it. I'd also use aluminum to save weight.
My big question is if it would be feasible to tie it into the bed for support. I don't know how much the plastic bed on the newer Tacos moves independently of the body otherwise you'd need some sort of elastic mount like say a muffle mount.
Just thinking out loud here..........
*If you don't mind having it look like a box inset in the bed wall. If you want it to blend seamlessly like that Nissan it's going to take quite a bit more work. That's not to say it can't be tasteful and look great to us, but it's going to look more utilitarian than the average consumer wants as it'll break up the smooth lines on the vehicle.