I know you said Zigbee, but if that's not a dealbreaker, the Zooz ZSE29 is Z-Wave and is sometimes cheaper than Hue (Zooz has sales on occasion, not sure about right now or what the stock is). Otherwise, depending on where you live and how sad you'd be if the motion sensor died, some people just take the risk of using "indoor" sensors outside. If it's reasonably protected, say, under a porch, even non-enclosed, you might be fine. I've also seen people coat the outside with a silicone spray to waterproof (and whatnot) things a bit or even coat the internal circuit board with a conformal silicone coating (leaving the battery terminals exposed, of course). Just depends on your tolerance for risk, I suppose. 
It sounds like you may have already tried something like that, so then there are a couple of other things to consider. If you can't face the sensor a different direction or "sheid" it a little bit from direct sun (these are pretty much all infrared-based sensors and will be tripped by changes in sunlight from time to time during the day), you could use multiple sensors and use something like Zone Motion Controller to combine them into effectively one sensor that only trips if a certain threshold is met.
Or, related to this concern from another poster:
Why not just "restrict" the automation the sensor is being used for during that time? For example, I have a sensor outside one of my doors that turns lights on with motion--but it only turns them on at night. There are many false motion events throughout the day, and...I just ignore them because they don't matter. (Of course, this is different if you're trying to use it for "a person is here." But then the idea of combining multiple sensors might work, or perhaps there is another way to reach that goal with something else.)
