Glad to see you looking to also combine some design elements into the treatments, as the ESL are stylish, so having them be highlighted by the space behind is a great thing to shoot for.
One idea I've shared before is to consider using the wire-frame and stretch knit fabric used for trade show booths and stand-alones to cover up either commercial treatments, or cheaper, stacks of wrapped acoustic treatment shaped to both fit, and meet acoustical goals.
While you can purchase those with any image printed on the fabric, a plain white will do, as one of the options is to put a Hue Light Strip, offset 6" or more from the fabric to light up the fabric and really frame your speakers.
If you want a U shape cuddling the speakers, then simply cut the U into a stack of Rockwool® Mineral Wool 60 Acoustic Insulation 48"x24"x4" sheets (get the 4" thick ones) and stack them high enough to read a few inches above the height of the panel.
Warp them in some light breathable fabric to keep the dust and particles contained.
Play with the light strips to see if one is enough in the shallow middle or two on either side do better.
Then use the wire-frame with fabric to hide all that.
I assume the U is because they go between the wall and some cabinet housing the TV and gear, right?
For inspiration, and some practical tips on DIY acoustical treatments, please read through my thread on my sidewall
Room treatments – part 2