Blow up dolls. They're easy to insert into the cavity of the door, then you just blow them up and TAADAA! Also hand for long cold nights on the road.
Honestly haven't done any insulating yet, so I'm following along to see what everyone else says too, then I'll just copy y'all.
That's both hilarious and brilliant at the same time!
I don't have a slider, but the previous owner stuffed fiberglass batting into every hole my van has.
I've heard that it collects moisture and causes rust, but that's not a problem I'm having.
It insulates pretty well, both sound and temperature, but I hate the idea of that scratchy stuff coming loose and floating around in the air.
I've taken a lot of it out, but it's hard to pull that stuff out through those little holes once it's been stuffed in there.