I think that you need a FLUSH MOUNT Toolbox
You want the tool box to sit back almost against the tailgate.
That poses some questions :
Does your Tonneau sit on top of the bed rails or does it sit flush with the bed rails ? You need to measure the useable height to fit the box.
Do you want to have any room between the truck bed floor & the bottom of the tool box ? So again height related.
Do you need the width to be 48" , which will fit between both of the wheel wells .
or
do you need the boxes width to just fit inside the bed rails , which is about 65" useable inches ..
which will bring up another issue $$
That leaves depth, from tailgate towards the cab ..
Is your cab a short bed or long bed ?
Allowing about 3" clearance from the back of the tailgate to the face of the toolbox, measure how far that you want the depth of the toolbox to be.
If the tool box is to be 48" wide, your tool box could go all the way to the front panel of the bed.
If on the other hand, you want the tool box to wider than 50"
The Depth of the toolbox will be impeded by the inner fenders.
( if you know how to weld aluminum, there is no problem. )
Say that you want the tool box to be 24" deep by 65" wide by 17" tall.
That is doable, but as you see in this image, the inner fenders are in the way !
i know, the image is of the type that sits on the rails, not the Flush Mount. but it shows that the inner fenders are in the way. the tool box won't sit where it belongs.
What has to be done, is to measure the part of the inner fender is in the way, what needs to be done ..
The corner of the box where the inner fender will not let it sit down, has to be cut out, cut it with a little extra clearance.
Cut both bottom corners to match, now the interesting part.
You have 2 sides & the bottom of your toolbox, the other corner is exactly the same.
Switch the right corner with the left corner & visa versa.
Now, turn either piece over & fit it to the corner, so what was the outside bottom of the box is now inside the box facing upwards & you now have a nicely notched box that fits over the inner fender wells with a bit of clearance.
I think i wrote it correctly.
Try it with a cardboard box, if you don't think it works
