I made a small version and I am in the proces of making the large one as we speak, and I picked up a few tricks along the way.
The pattern calls for 2.5" wide batting. It is actually better if you cut it narrower, say 2 1/4". You can either press it with iron or pin it. I am not a pinner but this time it worked for me. In a class I took after I finished my bag (I was impatient, what can I say) the instructor actually used that thin fleece product, I don't know how it is called exactly but it is much easier to work with.
Also, the pattern says to use strong thread to gather the bag on the top. Gathering is easy, sewing through it was impossible so I gave up on it all together. My handles gather the bag just fine.
I have to warn you though, you will get lots of compliments on your bag especially if you venture into LQS full of customers. Be prepared to be accosted ... I mean that in the most positive way. My small bag is planty big to fit a large wallet, iPhone, set of keys, small bottle of water and small totes umbrella without looking weirded. Very cool pattern. Good luck and enjoy. It is truly a fun project.