VEVOR Underbody Truck Box, 30"×17"×18" Pickup Storage Box, Heavy Duty Aluminum Diamond Plate Tool Box with Lock and Keys, Waterproof Trailer Storage Box with T-Handle Latch for Truck, Van, Trailer

I am using this box in a vertical position on my gate opener and it works fine. Box is thin walled and may not hold up vertically if mounted on a truck.

I vae mine sitting on the floor horizontally, the door is slightly smaller that the box on all corners so letting it sit vertically should not be a problem door will still operate

Yes it can,with the box on it’s side setting on the floor the door would be 1”off the floor

I would say yes, but I would remove the chain on the bottom side of the door so it doesn't get trapped in when closing.