The ones I'm most familiar with overhang the top of the bed sides. I notice you have some side rails on your bed, in addition to the headache rack. That would dictate what size you can use? Surely some of the guys here have these toolboxes.....???

Here's a link to quality boxes:

[www.cpodeltatruckboxes.com]{keyword}&gclid=CIyXkLCI7ZsCFShRagod-DiV_A&keyword=pickup%20toolbox

I know nothing about this company at this link, just one of many out there:

[www.truck-tool-box.com]

The latch is a simple pushbutton actuated hook that engages a bent loop, pushing down resets latch. A mill supply should have such an item? Also called a 'paddle handle latch' or pushbutton latch. Here's some links for stateside stuff:

[www.windcorp.com]

[www.lockmingyi.com]

[www.phoenixusa.com]

There's 2 choices on lid lifting: obviously springs, as in a coil spring lift, fairly simple to engineer. The other possible method is a counterweight: a false inner wall covers a mechanism of a lever arm which has a weight on the end. Unlatching allows weight to fall and lever flips top open. Moving parts, but no springs to fail, simple, last a lifetime. The lever, if connected in a straight line parallel to top surface of box lid, drops, with lid hinges at midpoint, similar to a see-saw or teeter-totter.