I realize that if I'm going to go with underfloor storage now is the time. Not later in the build.
There is a metal shop nearby who've I've worked with already on the build. They extended the tube on my new swing-out bike rack to clear the step bumper and were a pleasure to deal with ($20). So I'll go the custom box route.
I'd appreciate it if anyone who already has underfloor storage would give me any tips on the subject. Things they have learned or if doing it over what you'd do differently.
Here in no particular order are some of the questions I have at the moment.
Install a drain plug?
Install access door so it opens towards the front or back of the van. Doors that split in the middle?
What gauge steel?
How big would you make the access opening?
I'm not a off-roader so as long as the box is not lower than the bottom of the bell on the rear axle would I be OK?
How to mount the box? Make a box with flanges and bolt through the flanges into the van?
Or make a box with flanges then make the hole in the van just big enough for the box to slide through and the flanges then rest on the van floor? Or a box with no flanges where the bolts go through the inside top of the box and the van floor. See drawing.