Well, Dogboy asked me to do this a while back and the truth is that I'm pretty much a lazy old gaffer...but since I saw him request it a second time I figured that it was time I get off my flattened ass and get to work.
Aerostich's standard TF2 back protector is a piece of crap. It's too small, too narrow, and lacks hard plates. In a word, it's a sham to actually call it a back protector. Now while Aerostich does offer a decent Bohn back pro, it's a commonality that not all of us own it. In fact, it's quite possible that many of us own a different type of back protector that could well be retro-fitted into an Aerostich.
I just happened to have a Vanson 4-plate back pro in my garage and with a bit of manipulation it fit perfectly into my Roadcrafter jacket. I suspect it would also fit just fine into a Darien as well.
Step 1: own a Roadcrafter! On the inside of the jacket you will notice several female side velcro swatches.

Step 2: Get a Vanson Back Pro: here are two pix. Please don't mind the Duct Tape on mine...


Step 3: Please note that the Vanson also has female side velcro... So, go to your local Michael's, Hobby City, or fabric store and buy about 3 feet of 2" wide male side Velcro. Home Depot might even carry some. Cut it into four strips appx 8" long each.

Step 4: Attach velcro to Vanson Back pro as shown:

Step 5: Install Back pro by attaching the extra male side velcro to the inside of the Roadcrafter as shown:

Viola! You're done! Now put the jacket on before you go riding - no sense putting all that work into this and not wearing the jacket now is there?