Are you trying to do this in 34.11 or in 40.10? In 40.10, it should not be a problem at all. Just retire your fort (meaning retire temporarily, not abandon to ruin) and all the stuff will be pretty much exactly where you left it when your adventurer shows up two weeks later. I have done this a few times, and didn't notice that any items moved at all in the new version.
If you are paranoid, you can put it in a room sealed by bridges etc, but this isn't really needed. My suggestion would be just to put it somewhere convenient and easy to find near the surface, and have some of your military training nearby to prevent kobalds from stealing it in the time between when you start hauling it up there and you retire the fort.
If you are still using 34.11, then it is much harder. When you abandon a standard fort, all items will be scattered randomly among any regions with a clear path to the surface (including items which were in locations not accessible from the surface, such as those sealed by raised bridges etc). There are a few options:
1) Use DFHack to mark your fort as a lair. Then items won't scatter at all, just like in the new version.
2) Embark on a location containing an existing night creature lair. Kill the night creature (carefully) and place anything you want your adventurer to have inside the lair. Items inside the lair will not be scattered.
3) I *think* that item scatter is dependent on item weight, so that if you put the items inside of a lead bin, they will be moved much less by scatter, making them easier to find. I am not sure about this though.
4) As mentioned by DiacetylMorphine, if you forbid the item before it is removed from the workshop, it will stay there, and will not be moved by item scatter. However, I didn't think it was possible for adventurers to remove items inside workshops (could be wrong about this). To get around this, you can rig it up so that the workshop will deconstruct when you pull a lever (make the floor under it cave in).
Anyhow, if you're playing adventure mode at all, you should really try the new version if you haven't already.