Recumbent - Please recommend a seat bag for commuting.

I've got one of these:
http://www.hostelshoppe.com/images/products/la_10vol_hardshellbag.jpg

but it just isn't quite big enough. I can't fit a change of clothes in there along with a rain suit. I Would like something the size of a daypack backpack, and I'm thinking of just buying a pack with an internal frame and finding a way to strap it to the seat with bungee cords or something if I can't find bag that will otherwise work.

I tried my hiking backpack, but with a full load, I can't secure it high enough on the seat to keep it from resting on the fender, which then rubs on the tire.

Any recommendations?

Do you have a rear rack? I have used Jandd's waterproof grocery bag pannier. I put everything into a reusable grocery bag and it fits right into the pannier. When I get to work, I just pull out the grocery bag and head into work with all my gear. I am in the process or transitioning to a recumbent and haven't tried this on the EZ Sport yet. Good luck.

You might find something close to what you are looking for here. http://www.arkel.ca/ca_e/?___from_store=ca_f
Great products, great quality, innovative and reasonable prices. As well, some of their panniers and packs are designed for recumbents. Mike

There is no standard rear rack available for this model that I've been able to find. I'm still looking, since TW-bents used to make one, so there is probably one or two floating around out there somewhere for sale.

That Arkel bag looks like it might be just the ticket. My Volae bag is 980 cu. in., and that Arkel says it's 1100. I can just fit a change of clothes and my rain suit in my Volae, but I really have to cram it in there, and it makes taking lunch or anything else impossible, so I don't need much more room. Thanks for the tip.

Not sure which model you are referring to as 1100. The Tail Rider, by Arkel, is the one we use. It is 700 cu. in. But it it is huge and it expands the more you stuff into it. It also has a built in waterproof cover which is really cool and effective. Arkel has a new model coming out in March 2012 called the Tail Rider Expedition, which is much larger. Mike

A large lumbar pack (http://www.amazon.com/Mountainsmith-Day-Recycled-Fabric-Lumbar-Pack/dp/B001A4765W/ref=pd_sbs_a_1) might work, but you'll have to use your DIY skills to mount it.

I've tried a backpack. It works great empty, but as soon as I put some weight in it, it sags and rubs on the rear tire.

My google fu may have saved me, though. I found a vendor that used to carry TW-bent models in his store, and he might have some actual rear racks for the Elegance in stock, or be able to order one directly from Taiwan.

If the rack doesn't work, I'll probably order an Arkel.