I see that your speakers are JBL from the HK folks. Now let me guess, your television is a Sony. Even if it's not here's some information for you.
This solution uses flat 16 gauge sheet steel which is commonly available from Lowes, Home Depot, ACE and like stores.
(hint 1 - use heavy duty mounting tape to adhere the sheet metal.)
(hint 2 - wax (do not wax tape areas) the metal sheeting to avoid rusting.
(hint 3 - when the sheeting is used on the exterior of the speaker it may be painted to match the speaker.)
Check this out.
To add additional shielding within the speaker case: Open the case, determine the most appropriate location and take measurements.
To add additional shielding outside of the case: Determine the appropriate location (if not the entire side) and take measurements.
