New iNat merch idea!
Those magnetic signs that you can stick to your car. You could keep it inside the vehicle, but when you park, pull it out of your truck, backseat, etc and slap it on.
They are usually only 12" x 18" or 12" x 24" (about 30.5 cm x 46 cm or about 30.5 cm x 61 cm, for the metric folks), and very thin, so big enough to been seen but still very portable (for a car).
It could have the Nat logo and underneath say “iNaturalist”, or maybe a bit more.
eg. “iNautralist Observer”, “Citizen Scientist”, “Nature Cataloger”, etc