I’m wondering if a storm cover on its own, without hood or tonneau, would keep the rain out?
It does but it balloons into the car cockpit. I wouldn't trust it in heavy rain
Yes, I can attest to this. You end up with a big bag of water that is impossible to get off the car without spilling gallons of water either into the cockpit or down your jeans or both. It is a Bad Idea.
Tim H. 1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE