Hello Gree23!
Definitely create a build thread, link below, and we can add our feedback and comments as you progress. We love to see how our new members are progressing with their builds and are here to support.
https://forum.specops501st.com/forum/161-shadow-scout/
In terms of your question, the following are the specifics for the back armor:
Back Armor
The back armor has a center tank attached.
The top of the tank is flat with a detail piece painted black.
Top detail piece uses the same greeblies as seen on the screen-used ROTJ Biker Scout armor.
Top detail piece is painted matte black.
The tank has contrasting black stripe detail.
Optional: Additional "rank indicators" present on the right side of the tank, can number between 0 to 6 stripes.
OPTIONAL Level two certification (if applicable):
Back and Tank must be two separate parts, no single part back/tank plates will be admissible.
Only 1, 4 or 6 stripes on tank are acceptable.
Bullet tank on the back armor has accurate greeblie tank topper detail.
Tank topper is a separate greeblie to the tank itself.
Tank topper is secured with single black colored rivet in the center of the greeblie.
Rear tank has properly placed pin striping and a single bottom attachment black rivet.
Blinking lights or LED's on the bullet tank are not acceptable.
This is one of the areas that the CRL between the ROTJ Scout and Shadow Scout deviate. There is no mention nor requirement to have an elastic for the Shadow Scout back tank.
Additionally, for completeness, it should be noted that under the ROTJ Scout, the elastic, similar to the attachment methods above, are optional for level 2 only.
I hope this helps.