My husband - who used to be a boatbuilder and has seen a boat blown up by propane accumulating in the bilges (not one of his) - decided to build a propane locker for a 10lb tank in our Sprinter. We couldn't afford $500-1000 for a ready-made propane locker either. His is plywood sealed with epoxy, gasketed, and with robust pull-down hinges on the door for a good tight fit. It has a vent hole cut through the bottom of the locker and the floor of the van. Been fine for a couple of years with lots of national and state park camping. The only time someone asked was at the Brooklyn/Battery tunnel entrance. Luckily the tank wasn't in the locker at that point.
Someone suggested a 2nd vent hole to let air in and make the bottom vent work better, but we figure 1) propane sinks to the lowest spot, and 2) when we are moving we get a venturi effect with air rushing across the hole in the floor of the van, sucking out the air in the locker.
He even put air-tight glands in the top side of the locker so we can keep hoses set up in and out of the locker for a 1 burner coleman stove and our heater. We ALWAYS turn off the valve on the tank however, when we are not actually using propane. There is no propane in the hoses when the valve is shut on the tank. So this locker is just a way to safely carry the tank in the van that is also handy for use when we cook inside. We have a 2nd smaller locker in the back of the van, built the same way, with a 5lb tank for when we cook outside. That only has the bottom vent hole. If we had figured out what we were doing before making two lockers we would have made the first one - for inside cooking - for a 5lb tank and the one in back for the larger, 10lb tank - since we mostly cook outside. However, we did a month camping in the van every day last summer and didn't use up either tank. We usually cooked breakfast and dinner, but not lunch.
You CAN build your own locker, but we would urge a very serious consideration of any possibility of propane leaking/accumulating in your vehicle. Do everything you can think of to keep the locker air-tight and keep the tank turned off when not actually using it.