Concept: If you cave in the terrain the wagon sits on, it will be deconstructed and destroyed, but all of the items will survive in the hole you caved the wagon into. Thus: Drop the wagon into a hole, and then put a floor or bridge on top of it.
I started up a little howto game on an old world I had without an active fortress to walk you through the process. Double Biome, Terrifying Haunted Freezing/Cold Forest. Aquifer on the haunted side. Some metals I think, I don't remember. I don't pay attention to metals a lot.
Notable Notables:
Dwarves: 3 Miners, a peasant, a hammerdwarf, a mason, and a cook.
No animals (don't know if I get reanimate or not)
Embark stone options were fortuitous on this embark: Coal and iron and bronze.
*The world had 2500 embark points instead of the normal 1324, so I have 1176 to 'spare'.
3 Picks, I want to get underground as fast as possible.
This is what I arrive to:
I designate out my cave in area:
Note, a masons shop on the left and a meeting zone/pen for animals.
The undead horde quickly arrives by granite 2 however:
Cave in on granite 5th. I shortened the access channel for speed because of the undeads.
Alas, my efforts are in vain
Granite Tenth, Fortress Death. All the wagon items are on zlvl -2, I needed to finish my wall and my bridge for a seal.
I made some errors. I needed 2 more stone blocks for a bridge of that size, I needed to bring a training axe to make less work for my miners by cutting down trees which ended up slowing me down considerably, and after the cavein, I should have dug out more area below rather then having my miners idle.
But this is the general idea.