This is going to sound odd but the best device/aid that I have found to help me deal with sweat is a Garmin Varia rear-facing radar. Keep in mind here that I 'generate sweat from my eye sockets' so the help that I get from a Halo/whatever is limited.
In hot weather I always have a sweat rag (typically a cotton washcloth) in one hand or the other. For whatever reason my left eye is the more frequent problem (vs. right eye) so that rag is most often in my left hand. I have a rear facing mirror that clamps onto the very end of my drops. And you can't imagine how often I glance at that mirror and there is a sweat rag hanging across it. The Varia has really helped in that regard as I have found it to be very reliable.
dave