This extension will add up to 24" to your 2.4ghz antenna length by removing the existing antenna from your 2.4ghz receiver and snapping on the new extension. 
 
What you'll receive is:
a 10cm SMB straight plug x IPX /u.fl prewire
a 90cm SMB straight jack x unfinished end antenna extension
 
In order to use the wire, you'll need to cut the extension to length, then strip back 1.25" of sleeve and insulation from the tiny white center wire, which becomes your new antenna. This will also require running the antenna extension outside your waterproof cylinder and sealing that wire exit appropriately.