Most designs for dodge-me traps over long falls tend to be indoor with lots of mining and digging involved.
Why not build it outdoors?
Most forts after a relatively short time start getting buried in excessive rock, so use the rock!
Basically I'm thinking to create a secondary entrance into your fort, and building two towers with a fairly long bridge between them loaded with wooden junk weapons. Tower 1 takes enemies about 10 tiles up, then releases them onto a 10 tile long 1 tile wide floor with a cheapo weapon trap every tile (wooden spikes?) Tower 2 leads down to a very long path into your fort with lots of traps and whatnot that you can build at your leisure, or simply generate redirects that will eventually send the enemies back out.
Basically this would:
1) Allow building of a highly effective low tech defense without taking your miners off exploration mining, or metals gathering.
2) Use extra rock, rather than creating it.
3) Generate fortress wealth
4) Allow you to create a refuse pile under the pit trap, where the bits and pieces will rot away and not generate miasma.
5) Allow you to keep your dwarves from collecting the items by telling them to ignore things outside.
6) Reduce bloodsplatter in your fortress.
There's nothing to say that you couldn't, at a later point, install cisterns for hot and cold flowing destruction when you have time for a megaproject, or take it down because it's just too simple.
You could even build it from wood, in order to promote a proper relationship with elves.