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Might be a bit rough to get a camera in there that would focus that close...what is there maybe 6" from top to bottom?

Not at all. You shoot on a 45 degree angle like Rick's third photo.

Just curious, but if the M38 had a heater installed, would that interfere with the toolbox and modifications would have to be made?

For the hot water heater kit the only mod to the box is to run a fresh air inlet duct through it. They cut a hole in the front of the box's vertical wall and they cut the hole in the outside skin below the passenger door sill and run a hose between the two cut outs. Nothing to do with the lid or it's latch.