I only drew one fresh air/exhaust pipe, but two would probably work better, as long as the top one wasn't above the top of the glass, and the bottom one is aimed at the flame (so you can make sure it is still lit).
You want the top of the inner mailbox as air-tight as possible to keep the fumes out of your lungs, and allow the heat to transfer to the jumbo box.
The smaller mailbox lid needs to support the weight of a kerosene lantern, I figured a toggle (threaded rod with a wing nut on it)attached to the top of the arch, and a tab attached to the lid with a slot in it for the bolt to swing into should do the trick.
The jumbo mailbox should have two vents going into the TD, with a fan blowing the colder air into the bottom vent, any air leaks into the inner box would just blow more exhaust out the exhaust pipe.
I plan on mounting this in my tongue box with both lids pointing down. There would need to be a door on the bottom of the tongue box to protect it for travel and allow a kerosene lantern to be put in.
I figure mailboxes are fairly watertight, and with all the seams and holes sealed with high temp RTV they would be fairly airtight.
I'm not sure if an aluminum mailbox would be better than a steel or galvanized one, except that aluminum transfers heat faster.
According to the web, a kerosene lantern puts off about 1000 BTU's.