To create this item, you should have already scribed the book, Garik's Secrets of Bag Expansion which can be bought from Garik Nogflop (The Master Collector) in Sunrise Hills for 31pp 4gp 9sp 9cp, so that you have the tailoring tradeskill recipes and their trivials (appears to be all 15 and probably no fail combinations).
I used the guild hall loom but I suspect any tailoring kit having at least 2 slots will work:
1 x Unexpanded Tailored Legendary Haversack
1 x Abu-Kar's Arcane Solution
Trivial is 15.
NOTE: If you have the Unexpanded bag in one of your bags and the Tradeskill Window shows it is green and you hit the Combine button, it will balk and say:
Sorry, but you don't have everything you need for this recipe in your general inventory.
You are missing a Unexpanded Tailored Legendary Haversack.
Just hit the Experiment button and manually place both items in and hit the Combine button.
Edited, Nov 1st 2010 6:19am by Byren