I use 6.2V reverse biased Zener Diode (5 Watt) for Xenons. The 6V Panels that I’ve tested rarely reach 6.0 V, but this was only a 1W panel and the Xenon never Sleeps.
But Larger panels easily hit the 6.2V during full sunshine with the small load from a Xenon.
On a Xenon - Once the Li-Po voltage is above 4.05V, any disturbance in the Solar Panel Output ( a Cloud) stops that charge Cycle. A new recharge Cycle wont begin until the Li-Po is less than 4.05V.
Can you route the Zener to a Jumper that allows the user to remove for Borons ?