Tryplay - Bike looks fantastic - I like the Black & White theme, the Flo wheels work perfect with the ascetics of the bike - Good Job!
Here are some pic's of the Bent Aero Bag and how I made it work with the stock seat.
This is the piece of plastic - not sure what type, but I simply cut the piece out from an irrigation box from Home Depot. I used heat over my stove to make the curve. And then Velcro'd it the sock seat to form an extension of the existing seat to get the bag to fit. As if the seat was your standard hardshell in which the bag was intended.
Here is the extension Velcro'd to the stock seat.
Seat pad covers extension somewhat - this is a Ventisit pad
Mockup to show bag is now filled out and can not slide down. I zip tied the top of the plastic extension to the neck rest top for more stablilty
Hera is the plastic extension with the bag and 2 holes drilled to zip tie to neckrest tube
And a really bad photo from iPhone with everything in place. I used a piece of open cell (cut from stock Bacchetta seat pad) foam behind the seat pad to give me a little shoulder rise, but not necessary to make extension work with stock seat.