Aspire Turbo Case X-Sonic. Very like the original Antec LanBoy inside and will fit the LanBoy carrying strap. About 10 lb w/o PSU. 15lb Gross shipping weight including 350W PSU.
Link .
. Even this one has only .8mm Aluminum which will be pretty flimsy unless it's high quality, relatively stiff Al-Mg alloy (often called "aircraft grade"). The Aspire Turbo Server would work and is stronger (1.0mm Al). It is also about 15lb Gross with 400+W PSU. The PSU in the X-sonic is a L&C bottom feeder - dump it to a noob on eBay (or closet it for an emergency backup) and put in a real psu. They try to fool you with the PSU in the Server as the model begins with CWT (one would think Channel Well Technology, right - well, one would be wrong...

). Made by some unknown co. called Real Power etc. Handle this PSU as above.
I doubt you will find anything lighter that would be at all sturdy - though some out there are .6mm Al. IOW, made from heavy-duty Reynolds Wrap...
Some of these are available in the Chieftec or Chenming brand w/o PSU, but the price diff isn't much - a testament to the "quality" of those PSUs. :roll:
Enermax has the CS-517AL (and others with diff fronts) that I would estimate weigh about the same (can't find the spec on mfr web site) and are around $50. w/o PSU. These don't have any modern conveniences inside like drive rails, removable bays, etc. which the Aspires have.
All are available in at least Silver and Black and any with a window may weigh a bit more.
Some Skyhawk Al cases are being blown out for around $25. here and there - about what they're worth (some of those are .6mm)... Check the Newegg clearance section.
.bh.
:moon: