I think the "best" option would be to fit the assembly of the cabin with everything straight & tight, then do as was suggested & shim to the frame. You wouldn't need steel wedges (too complicated & difficult for tooling setups) but some hardwood wedges would work if they're fastened with a good bedding adhesive to the bottom of the deck & treated for water (epoxy, roof coat, asphalt emulsion?).
My build has a bit of the same thing, so I just ran the aluminum sides down a small amount past the walls & trimmed them out. Used the factory edge as a straight line & it's all good.
Here's a shot of the side with the edge molding set in place. It only hangs down about 3mm past the bottom of the deck, but it's enough to make the edge look good & true. It looks like it's hanging down more, but that's only the 2x4 sleeper the box is sitting on & angle of the shot.

Once it's dropped onto the frame, who cares & who knows....