What did you finally decide on?

The two heatsheets idea sounded like a straightforward one to me. Before they had heat sheets type bivvies, I used to take two space blankets and tape them together on three sides to make my own emergency bivvy. I used clear mailing tape. It seemed to work fairly well. I wouldn't think being too wide would be a problem. I would think that being too narrow would be a problem.

HJ