Another poorman's tarp are the plastic covers used to ship mattreses.If you have a local retailer with a warehouse they will have rolls of new bags on hand. A smile, $5 and you have thick mil, opaque plastic twice the size of a double/queen.kingsize bed.
A tarp is a excellent item to pack against the back for comfort.
You will save valuable time if your tiedown knots are pre tied ( there IS a tensioning knot, or use the plastic attachments)and at least a few groundstakes.
Fire and tarps- make them easy and quick. Once inside and warm your brain can nitpick 'shortcomings.'