To answer your first quesiton, very dense and thick foam (properly attached too to prevent vibration transference - as in, not just "set and forget" flopped ont top of the car) should do the trick. That's effectively what the Rycote rain covers are in a general sense - thick and dense over-sized foam to help absorb the rain drop sounds from transferring to the windjammer it covers. I'm not sure how much foam you'll need, but you may want to test if you can beforehand.
Here's the secret too: rain will get added in post. I'm not saying capture it noisy, but if you can't quite baffle all rain 100%, don't fret too much. Any baffling you accomplish, even minimal, is better than none at all. The dialogue editor will build out the necessary fill to match the residual noise, and then you'll be adding more tasteful rain back in with an FX/BGz pass. You'd be amazed how horrid some rain scene dialogue sounds in it's naked form (I've certainly heard my share), especially once in the mix it realizing it passes just fine because of the properly-build underlying rain backgrounds and just the right amount of premix magic makes it pop nicely without too much effort.
If anything, the MOST important thing (even above noise baffling) is to record consistently and appropriately for the scene and the editorial needs. Even bad/noisy dialogue, when at least recorded consistently and with care taken to axis and coloration, makes such a scene a whole world easier for a dialogue editor to cut. Consisistent and predicable noise across a scene (and across multiple mics) recedes into the background psychologically. Poor recording considerations (even in clean, noise-free situations) will do the exact opposite.
Noisy dialogue recorded consistently and with good solidarity will blow squeaky clean but inconsistently and poorly mic'd dialogue recordings out of the water any day. Noise isn't all that bad if you respect it. This is unfortunately where many a production mixer fall, and once it gets to the dialogue editors hands it's already too late to respect 'noise'. What was that Jurassic Park line? Something like "I was overwhelmed by the power of this place, but I didn't have respect for it, and that power is out now"? Same idea here with the concept of 'noise'.
I share my two cents from dialogue editorial experience (and some minimal production sound experience).