I'm going to steal your roof mount idea! 3" angle raw material?
Also your use of wall space for the inverter and electronics!
I'm pondering the ceiling, too. I also don't want to lose any headroom, so I won't be mounting anything to the bottom of the ribs. I'm currently thinking of scribing 3/4" thick material to each side of each rib, that would in turn be a ledger to support 10-12mm laminate flooring with .5" of insulation between the laminate and the ceiling. The laminate "ceiling" would be about an inch above the bottom of the ribs, the ledger would be maybe .25-.5 above the bottom of the rib. I'm not to concerned with seeing the rib and it's associated holes, I can always cover them with white rubber traction tape, available at Marine Supply stores or Ebay.
2015 T250 MR cargo w/windows, LS. Purchased new 9/2016. Richmond, CA
Go Ducks! track&field 84-86