You basically want to replace the back seat for a trackday with a fiberglass cowl?
Could I recommend this instead: buy a replacement undertail/battery tray. With a jigsaw cut off the 'carbon fiber look' parts, leaving the battery tray and ecu mount, reinstall. Use an ArmorBodies or similar Superstock Race tail - exactly what you want (you have to trim the sides off the undertail to get the fiberglass tail to fit), and you can then mount a foam block backrest.
You'd then switch between the setups (yes, moving the battery tray, battery, ECU, and starter solenoid before/after trackdays).
Here's what I mean:
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I didn't bolt the race tail on for the photo so it's kinda dipping down under the seat, but you get the idea.