I have a similar problem with my Garmin Varia radar. This is exacerbated by having short legs, so my seat post doesn't have much exposed length to it.
On one bike, I got a real small saddle bag and crammed it under the saddle.
On another bike, I bolted a Garmin flat mount to a rear luggage rack, which works fine if you don't mind having the damn rack on all of the time. (I don't, because I often use it, but lots of roadies wouldn't want to to that).