Does anyone have suggestions on the best way to protect the 830 in heavy rain, without messing up the altitude/elevation detection?

Locking the screen because it’s raining or the screen is wet it could be a solution. But, again, this is a proof that Garmin is aware of the bad quality screen and give us stupid solutions that are unacceptable for the product price.

I agree with this 100%. I have had Edge devices going back to the 510.  Rain was never an issue.  I went through 2 530s already.  With the sensor at the bottom of the unit not only will rain get to it but sweat as well.  I ride MTB and mount right over the stem for protection.  This brings the unit right where my head drips on climbs.  As soon as a little sweat gets on it the elevation quits tracking.  I tried many times to replicate and or not replicate.  Every time it gets wet in this area elevation quits.  If I mindfully keep it dry it tracks perfectly.  This is really disappointing.  God forbid a fitness tracker intended for use in the elements gets wet.  I am selling mine at this point and getting a Wahoo Roam. 

I have had the same problems with my 830. After reading this post I unscrewed the USB flap and what did I see? Wet inside  and the pressure ports are "filled" with water. I did have the same problems with my 800 many years ago when it rained and the fix was to suck out the water from the pressure port. Now I will let the 830 dry out and hopefully it will be back as it was for the first 18 months.

Just did the "suck"-test after probably most of the humidity had dried and suddenly OPS Normal. I will get back on this chat if its not ok on next ride.

Just came across this post today....I have just had very similar issues with my new 1030+. On my second ride with the device in constant rain...the Grade started showing ‘0’ or ‘-1’ on a climb that I know is around + 5%. Also then noticed that my Total Ascent had completely stopped accumulating. At the same time I also was aware that the audio ‘beeps’ from the device were non existent. So it seems both the Barometer sensor and also the speaker both became waterlogged...just by riding in rain! 
I returned the unit for a replacement and also emailed Garmin to make them aware of the issues. The reply was not encouraging...”We will send out a rubber bumper to protect it in the rain.”  I stated that these devices are IPX7 water resistant and that this should not be happening ....still awaiting a reply to that. 

Maybe a minor point but its IPX7 water resistant (protection from getting damaged by minor temporary water immersion) is irrelevant to the problem.  The unit isn't getting damaged.  It's temporarily incorrectly reporting the elevation because the baro sensor needs clear exposure to air pressure, which it doesn't have when the port have water in them.  That's a different problem and not related to IPX7 certification. 

Thanks.  Good to at least know I am not alone.  I could wrap in plastic wrap but would be concerned that would mess up the barometric pressure reading.