under the seat works if you put it in a slide out with extra wire so it can slide in/out
behind the seat works but not easy to access even if the seat folds ahead
under the dash works on a hinge/latch so it is accessible easily but out of sight. again, a little extra wire looped up there to allow the panel to move easily.
if you have a console it could be placed in there, just a little harder to access
I like to keep the panel on the drover's side so when powering on/off the key-checking circuits- you have the key within arms reach when you are hunkered down on the drivers floor under the steering wheel
a small light could also be placed next to the fuse panel, like the older chevy trucks had that mini cord reel under the hood with the magnetic light back in the late 80's early 90's
to get the wiring through the firewall or floor ensure you use a weatherproof connection so if it is wet outside it isn't also wet inside the truck. you could repurpose a stock bulkhead connection or iuse an household electrical compression fitting. a rubber grommet works but they also leak, usually, unless well sealed when you're done all the wiring.
I have seen some 4x4 guys use tupperware boxes and those household electrical compression fittings to put electrical stuff inside. the box is mounted with sealed screws from the inside, these also hold the fuse block, then the wires are brought in through the side with sealed compression fittings, then the lid is snapped on for a leak free junction box
if you google sealed automotive fuse panel you will get some hits, sma e for bulkhead connectors
anyway, just some ideas
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